Sometime in life, we all ask some basic questions, and once asked we make at least some attempt to seek and search for their answers. Questions such as:
- What is the purpose or goal of life?
- What is this world and who, if anyone created it?
- Who am I?
- What is my relationship with the Creator, the world and the others?
- How can I be happy?
Science, Philosophy and Religion have made, and continue to make, serious attempts to answer these questions; and make further attempts to question the answers thus produced. Quite a few answers produced by religion are beyond the scope of science. Religions depend upon faith to derive the answers which they may term as 'spiritual truths'. These truths must invite seekers to verify them for themselves through experiences. These answers naturally cover the universal aspects as well as short term and long term necessities of a spiritual life.
Sikhism is one such modern, religion propounded by the ten Gurus. First of them was Guru Nanak who started preaching in 1499 in (Punjab) India. Being a compassionate person, Guru Nanak had observed the selfish ways of life and was touched by unhappiness prevalent in people at large in his time. He saw the tyranny of Muslim rulers . He commented on the weaknesses of both ' Hindu and Muslim ' communities. At that time the noble Hindu culture had, in practice, yet again, fallen a victim of decadence . Casteism was yet again rampant and lower caste persons were treated inhumanly. Mechanical performance of rituals had become the norm. Internal purity of mind and spiritual progress were not the aim, only external actions were the aim. Despite the divine knowledge about the four Paths for 'Spiritual Progress' viz. 'Jnaana Yoga' (Knowledge) , 'Karma Yoga' (Action)  ', 'Paatanjali Yoga'  (also known as Raja Yoga) and of 'Bhakti Yoga' (Devotion) being available, upper castes including Brahmins, in their selfish interests, were inhumanly exploiting the lowest castes. On the one hand such divine knowledge was available, and on the other such abysmal inhuman behavior!
Bhakti Movement : Historical Background
It would help to understand the evolution of Guru Nanak's devotional path - 'Naam Simarana' - chanting the Name of 'Advaitic (non-dual) God'. Among many others, the old scriptures like Bhagwad-Gita, Upanishads (Upani) and Naarada Bhakti Sutra (NBS) and even Paatanjali Yoga Sootra  have discourses on paths of devotion. Bhakti (devotion) basically is remembering and chanting Names of Gods or Supreme One. NBS is a comprehensive treatise on paths of devotion, inclusive of chanting of His Name, and all other modes described in Guru Granth Sahib (GGS). Bhakti (devotion) movement in India is ancient. Both Patanjali and Naarada are believed to belong to the ancient period, but certainly a few centuries prior to Christ. Bhakti Movement of 'Alawars' was prevalent from 7th to 9th century A.D. in South India. Around 800 AD, the greatest exponent of modern era of Jnaana Yoga, Aadi Shankaraachaarya (Shankara) himself had written and sung devotional hymns; and he, a brahmin, had accepted an enlightened lowest caste person as one of his Gurus. During 11th and 12th centuries in South India Sant  philosopher Raamaanujaachaarya (1017 - 1137) expounded 'Devotion' as per his 'Qualified Monism'. He indeed had rebelled against the caste system of Hinduism. He, a brahmin by birth, accepted an enlightened person of a lowest caste as his Guru! And he preached the lowest castes openly.
In north Karnaataka, Santa (Saint) Basawannaa, also a Braahmin by birth, had propounded an adwaitic (non-dualist) Bhakti movement in mid 12th century, through his devotional songs known as 'Wachanas'. He propagated that in the spiritual path all humans are equal regardless of their castes, gender, class and status. He was one of the greatest rebels, if not the greatest, who not only rejected the caste system but also the important concept of rebirth. In his time a galaxy of talented women, the most courageous being Akkaa Mahaadevi, became rebel devouts who were venerated as sants. Santa Jnaaneshwara (1275 - 1296) of Maharashtra, in the 13th century was highly respected preceptor.
Naamdeva (1270 - 1309), also of Mahaaraashtra, a lowly tailor by caste, enriched the Bhakti tradition with his both Saguna (God with form) and later Nirguna (God without form) devotional songs. In Kashmir, Lalleshwari (1335 - 1376) propagated 'non-dual devotion' through 'waakhs' (sentences). She, living under a Islamic ruler Shah Mir, proclaimed, 'Supreme One pervades the world, Hindus and Muslims are the same.' Devotional Movement was brought by Santa Raamaanand in the mid 15th century to North India. He openly sang, 'Nobody asks for anyone's caste, for one who chants His Name becomes His .' He also propounded a concept that although God Rama was a reincarnation of the Formless  One, He is the Supreme Spirit the Formless One. And his disciple Sants Kabir (1440 - 1510), Ravidaasa (15th century), and Dhannaa (15th century) etc. were preaching 'non-dualistic (Advaitic) devotion'. Ravidaasa was the Guru of Santa Meera (1498 - 1563). (Akkaa Mahaadevi, Lalleshwaree and Meera form the trio of famous women rebel devouts that the Hindu society produced 7 to 8 hundred years ago). Guru Nanak (1469 - 1538) also taught the same , and used all other names of various reincarnations of Brahman like Hari, Har, and Gobinda etc. to convey that these Names, commonly used to indicate different Gods, in fact, indicate the same Supreme One. 'Chaitanya Mahaaprabhu (1468 ' 1533) of Bengal, a contemporary of Guru Nanak, was extremely respected in North India, with his Dualism-based Devotion.
Vallabhaachaarya (1479 -1531) and Shankar Deva (1499 - 1569) were extremely respected with their 'Duality based Devotion' in the 16th century in North and Northeast India respectively. In that period the Bhakti Movement spread in the four corners of India. It also refutes the claim of some writers viz. that India was never a nation before the arrival of British. India was a 'nation' in a deeper sense than in a mere political sense. India was and is a 'Nation' united in every Indian's heart. All these devotional movements are still prevalent in India (unfortunately, except in Kashmir), e.g. Chaitanya Mahaaprabhu's Path is being followed internationally by 'Hare Raama Hare Krishna Movement'.
It is a truism that there are always ups and downs in peoples' lives and cultures depending on vicissitudes that they pass through. From this extremely brief view of development of 'Devotion', recurrence of casteism and rebellions against it in Hindu Society appear to be one such phenomenon. What is worth noticing is that Hindu Dharma is a highly dynamic way of life, from which reform movements spring whenever there is a need to suit the changing social, economic and philosophical conditions. It may also be worth noticing that most of the rebellions against Caste System were initiated by Brahmins. Guru Nanak also belonged to an upper caste.
Guru Nanak (1469 - 1539) was born into a Bedi (well versed in Vedas) family, in the village Kartarpur in Punjab. He was brought up in the Hindu tradition of 'Vedic learning '. Guru Nanak was well versed in most of the Hindu scriptures, and had reverence for them . From among the four 'Paths', Guru Nanak realized  the non-dual God, the Supreme One, 'Parabrahma' early in life through the path of 'Devotion', and he got the answers to the above basic questions. He realized the One Reality, and the utter unity not only among humans but in the entire cosmos. He decided to make the world a better place, and started preaching the Truth, the way to ultimate happiness, contentment and bliss. Guru Nanak did not see different  castes, genders and status as different for spiritual realization as had become customary among the calcified Hindus. To him not only were rituals useless but, if performed perfunctorily , were obstacles in the path of realization. He was against  those rituals which either did not purify the mind or were performed superficially . He was not against the rituals that would purify  the mind. He wanted the heart to be as pure as possible to begin Naam Simarana, which would further purify the heart till realization takes place. Like a realized person, he worked for happiness of the whole humanity.
The great genius of Guru Nanak was that he :
Simplified the process of devotion (by chanting) to the utmost and named it 'Naam Simarana'. He had realized that understanding of Vedas esp. the 'Advaitic philosophy' of Upanishads for a man of the world was difficult, as availability of teachers had become difficult. This achievement of simplification is even more remarkable for he did not lose the philosophical essence of complex hymns of the Upanishads.
Established the practice of group chanting daily in a regular and disciplined manner. Thus he developed social harmony and love. To avoid rituals, he started a separate temple, appropriately known as 'Gurudwara', ' door of Guru - for congregational chanting. He used the language of common man like some other Sants. He also used Sanskrit for a few Shlokas (couplets).
He started teaching congregations his message through devotional songs (not just poems but musical compositions), obviously one of the most attractive and effective methods. This was done for the first time probably after the compositions of 'Saama Veda' and some Upanishads.
Although previous Sants had written devotional poems, they had not got them composed musically.
In Gurudwaras he started the custom of distributing sanctified Karaaha Parsaada, a sweet food (halwa) which is prepared in a Karaaha - an iron cauldron, hence the adjective. In a congregation, while distributing parsaada, no differential treatment was given to any person based on his caste, gender, status or class. This was a very simple and yet extremely effective method of ensuring equality of all before the Supreme One and also in the society.
Those who accepted his teachings were known as 'Sikhs'  literally meaning disciples. In the period of early Gurus, Sikhs basically remained Hindus. After considerable time the word 'Sikh' evolved into the meaning that we are familiar with today, and 'Sikhism' was established as a religion.
Dualism and non-dualism
The goal of both Sikhism  and Hinduism is to achieve happiness here and now and also to attain 'Moksha' hereafter i.e. liberation from the cycle of birth and death. Looking at the complexity, and difficulty of the other three Paths or Yogas, Guru Nanak chose the simplest 'Path of Devotion'. In this Path there are three ways:
- Dualist Path of devotion : There is the God and separately there is His creation. He gives His grace and is merciful, but no person's soul can ever unite with the God. Some dualists believe that He has a form (saakaar or saguna i.e. Brahman with form).
- Non-dualist Path of devotion: There is the Supreme One who is formless (Niraakaar or Nirguna ), and there is His creation, but in essence both are the same. Any person's soul can unite with Him, indeed the soul is the same in every one, and the Atman and Brahman are the same.
- Dualist- Non-dualist Path: The Supreme One is indeed formless, but He also takes forms when needed to restore justice. This path believes in 'awataarawaada' (the other two paths do not believe in this).
Out of these, Guru Nanak chose the Non-dualist (Advaitic or Nirguna) Path, which had been already used by Santa Naamadeva and Santa Kabir etc. who had been preaching the non-dualistic (Advaitic, Nirguna) Path for the past 200 years. And what I find extremely praiseworthy is that Guru Nanak deliberately uses the names of Hindu gods with forms (Saguna) in far too many places for it to be either a chance or to meet necessities of rhythm for the song, or merely to please any group. Names of Gods like Brahma, Vishnu, Mahesh (or Shiva), Rama, Gobinda, Krishna, Paarwatee, Har, Hari etc., who all have 'forms', have been used liberally. There is an apparent contradiction in believing in the Supreme One who is both 'formless' (Nirguna) and with form (Saguna). This does not throw any doubt on his firm faith in and experience of the non-dual ('Nirguna') Supreme One, but indicates that he is preaching that ultimately the Truth or Parabrahma (Supreme One) is both, formless and with form. Thus rather than merely condemn dualists and create a confrontation, he lovingly reconciles  the difference. He says that Rama, Krishna, Shiva etc are the same as Parabrahma. Indeed Hindu scriptures keep reminding everyone about the same truth. 
Scriptural Sources of Sikhism
Sikhism has two authoritative scriptures which are authored and edited by the Gurus viz. Guru Granth Sahib (GGS) and Dasam Granth (DG). DG is authored by the Tenth Guru - Guru Gobinda Singh. DG is also a rich source of information on Gurus and the structure of Sikh Panth . However, most of the philosophical bases are in GGS. Superficially, GGS appears to be an anthology of 5894 devotional songs by the ten Gurus and some Bhagats (Sants) , but it has a meaningful structure. It opens with a 'Moolamantra' (Moola means seed or fundamental, a mantra is a capsule of spiritually powerful syllables). The Moolamantra is not really a definition of the One Reality but indication of some of His attributes, although He is attributeless. It also announces that He is realizable with the grace of Guru. This mantra is expounded with its philosophical bases in Japuji, the next small section consisting of 93 pages out of the total of 1429 pages of GGS. The philosophical bases are further expounded in the rest of 1336 pages. Some appropriate references from Hindu scriptures are also given by me to indicate their relationships.
Faiths in Sikhism
Faith forms the basis of any religion. However very often faiths, till the end, remain faiths. But that is not the case either with Upanishadic or Sikhism faiths. Guru Nanak asks his disciples to first verify the path and then follow it . Main faiths  of Sikhism are the same as that of Advaitic'Devotional Hinduism, and they are:
There is only one Reality , one Truth, one Absolute, the Supreme - One. He is Infinite, Formless, Never-changing, Unfathomable, Inaccessible  etc. There is only one Supreme One i.e. One Onkaara, and there is nothing else . 'He', the Supreme One I am ; all this (world) is That  (Supreme One). All this (world) without That (The Supreme One) is like a dream . Activities of the world without the Supreme One are all false  because they are like a wall of sand.
Ik-onkaara  (the Supreme One) is transcendent and immanent . He is transcendent because He is beyond the reach of body, mind and intellect. One can achieve the transcendent state of consciousness, which is beyond the normal state of consciousness, and in that state one realizes this Unity. He is immanent because He pervades all His creation. The transcendent state has been called as 'Sahaja ' in GGS. He is the eternal Being who becomes this cosmos.
Ik-onkaara is ineffable Brahman  the Supreme One is beginning-less, formless, infinite  and changeless. He permeates everything and yet cannot be seen, He is the Truth of Truth , He does not do anything, yet nothing happens without Him. These are the ways in which That ineffable is described . He is beyond description , because he is beyond experiences of mind and senses. He is realized in the fourth state (the transcendent or 'turiya') of consciousness, beyond the three distinct states of consciousness viz. awake, dreaming and deep sleep.
Rebirth  as per Karma Siddhaanta  - The law of causality states that all causes result into effects and all effects must have had a cause. Therefore all actions performed by an individual must result into their effects. All causes in one's life may not be able to effect in their results. Therefore all must be reborn again and again till they can exhaust their karmas (actions). If a person is born in to a noble family, it must be an effect of his noble actions in his past life for there is no valid cause for him in this life responsible for his birth. All have to pass through petty pleasures and pains, successes and failures, happiness and miseries etc in the continuous cycle of birth and death.
'Moksha'  (liberation) - Moksha is the highest goal of life on attaining of which one goes beyond the cycle of birth and death for one unites with the Supreme One who is Bliss personified. For happiness here and Moksha hereafter, Guru Nanak says follow the path of devotion. Both, happiness here  and Moksha hereafter, are attained by realization of the Supreme-One (Ik-onkaara) through the path of devotion ' of 'Naam Simarana' i.e. remembrance of His Name. This state of realization is also described as one in which one receives 'amrita' or 'ambrosia ' that is one becomes Eternal ' passes beyond time.
Path of devotion and need of a preceptor i.e. a Guru ' The Supreme one (Ik-onkaara) can be realized through devotion, but not by thoughts , rituals  and external symbols. In this path a devotee chants the Naam (name), with total love and devotion to Ik'onkaara who is formless . Grace of Guru  is essential in this Path of devotion i.e. Naam simarana i.e. remembering His Name. Guru  is either Ik-onkaara Himself or a person who has realized Him . The two sacred epitomes - 'Aadi Granth or Guru Granth Sahib' (now onwards GGS) and 'Dasam Granth' have also been given the stature of 'Guru' by the tenth Guru - Guru Gobinda Singh. Devotion to Parabrahma has to be absolutely exclusive , as was the love of gopies  (Milkmaids of Brindaaban who loved Krishna absolutely exclusively).
How does Naam Simarana path of devotion work? Path of Naam Simarana lifts a devotee's mind beyond desires and thoughts into his Self. Thus Naam Simarana changes the outlook of a person on desires, indeed on his purpose of life, etc. It brings his ego, anger, greed etc. under his control, makes his mind pure and ultimately unites him with Brahman.
Maya  - If a man is Ik-onkaara (Absolute God) then ordinarily why doesn't he feel or know accordingly? Maya is the creative power of the supreme One by which He Creates this universe. Man is created to live and work in the world so ordinarily his senses have to look outwards into the world of objects . But a man gets totally absorbed in desires and objects of the world, thus he forgets his true identity, and falls a victim to 'duality', which prevents his realization . Duality is the belief that spirit and matter are two totally different entities, and other than Brahman (Pure Spirit) there is existence of creatures who can never unite with Him. (In Hindu scriptures there are concepts of 'vidyaa' and 'avidyaa'; and avidyaa is caused by Maya). It is due to Maya that a man thinks that he is this mind ' body ' intellect. A man who identifies his Self with body etc. cannot attain infinite Bliss. Indeed such a person is going to suffer here and hereafter. Guru Nanak has invented an appropriate word for such a person suffering from duality, under the influence of Maya 'Manmukh '. And for a person who does not identify his self with his body etc. and is devoted to Brahman, Guru Nanak has invented the word 'Gurmukh '. (Maya is the cause of duality and avidyaa.) Maya has many meanings - power of creation of Parabrahma; power that creates duality i.e. delusion or illusion in a person's mind; this duality or delusion, or illusion itself; of the nature of the three Gunas  (power of action that binds). All these meanings have been used in GGS.
Equality of human being and necessity of 'character'. Anyone can realize Brahman regardless of his caste, class, color, gender, occupation, status etc. This path of devotion, however, does not work for persons who are not truthful, have no character, are not kind, and have not controlled their ego, desires, senses and mind . But such persons are not lost provided, while walking this path of Naam Simarana, they do so with whole hearted sincerity for then such impurities get washed  away. Serving humanity purifies the mind and thus prepares one for Realization. If every person is equal before Parabrahma, the Supreme One, then there should be no ill treatment of any creature especially of lower caste people and weak people including women! Not only are all human beings equal before Him, but they, who are devoted to Him, are also Him . Whosoever chants His Naam with loving devotion, he gets bliss and moksha. Brahmin (the superior caste) is one who has realized Parabrahma , and not the one who is merely born into one.
Sikhism is Monistic
All the authors that I have read on Sikhism consider Sikhism to be a monotheistic and a revelatory religion. In monotheism duality , meaning existence of two distinct entities, remains - duality between one God and the rest who are different from God. Humans may receive His mercy or grace but cannot unite with Him. But Sikhism, like non-dual (Advaitic) Hinduism, is a monistic (Advaitic or non-dual) religion. As per the Guru's precept there is nothing else other than the Supreme One. Monistic or Advaitic path is also called 'Nirguna'  (formless, opposite of Saguna (with form) path. Some persons see no difference between polytheism (also pantheism) and monism. In polytheism there are many gods, as sky-god, sea-god etc. In pantheism all forces of nature are gods. In monism everything is the same Supreme God, sea-god is Him and the sea also is Him, whole nature is His creation and is, in essence, same as the Supreme One. All that exists is Him. The differences we see are differences in mere names and appearances, like all gold ornaments are but gold having different forms and names. We are not aware of this unity because our consciousness is covered with desires and thoughts. As soon as a person realizes the Supreme One, these differences disappear and he becomes Him . Therefore monism is not polytheism or pantheism either . The Supreme One (Absolute Formless) is all . Therefore Sikhism is neither a monotheistic nor a polytheistic nor a pantheistic, but a monistic religion.