For the following reasons Guru Nanak was a reformer and not a revolutionary.
- Nanak opposed the caste system but it was so mild that it never attracted any opposition from other castes.
- Nanak’s denouncement of fasts, penances, pilgrimages implied attack on perversions and not on the basic tenets of Hinduism. Fasts were only a means to an end and not an end to themselves.
- Nanak never depreciated the Vedas. He criticized the blind and mechanical reading of these texts without realizing the God through them.
- Even the Sacred Thread was not wholly condemned. He denounced the ignorance of the implications of its use.
While challenging the predominant position of Hindu deities, Nanak did not hold them in any disrespect. He wanted people to realize that they were not above God.
- Nanak suggested no further change in the civil institution of Hinduism.

In short, Nanak was a reformer like many others before and after him e.g. Basavanna (link at end of article) in Karnataka. The concept of love and devotion preached by him found wide acceptance. Khalsa was the result of Mughal tyranny. The five K’s were used to identify people who supported Govind Singhji's fight against the Mughals.

Here is why the author considers Sikh Dharma to be a part of the larger Hindu community.

Dr Arvind S Godbole wrote in The Unbreakable Hindu-Sikh bond, "To qualify as a ‘separate religion’ it must have a theology and philosophy distinct from other religions. The revered, Shri Guru Granth Sahib (Granth, G.pp. no) is the most important source of Sikh theology and philosophy. According to the Granth, the supreme Being is sans beginning (G.1351), primordial being (G.129), complete or integral (G.705) eternally true (G.1,119), sans human birth (G.1,99), transcendent as well as immanent (g.79, 102 etc), antarjami (G.13,43,454 etc.) nirvairu or sans enemity (G.1,99), fearless (G.199), fearless (G.1,464 etc.), supremely resplendent ( G.13,277 etc.), supreme bliss (G.814), untainted or niranjana (G.119,597,1353) and both sarguna and nirguna (G.128,862).

These basic theological concepts are of the Sanatana Hindu religion. Shankara in his Vivekachudamani (225) calls Parabrahma as nitya or eternal. Bhagvadgita (9.18) regards the supreme as the primordial origin of the universe. The Chandogya Upanishad (8.3.8) holds that the truth is His name. Bhagvadgita, (7.25), declares that the ignorant think that the Supreme Being has a birth. The immanence of the Supreme being, a cardinal tenet of the Sanatana Hindu religion and the Sikhism differentiates then clearly from the Semitic religions, who do not subscribe to that doctrine.

Several hymns of the Granth, bring out very eloquently, the contrary attributes of God e.g. ‘You are the teacher, you are the disciple’)G.69); ‘You are water, you are the fish’ (G.85). This is a corollary of the doctrine of total immanence of God and is an echo of the Taittiriya Upanishad. Antarjami (antaryamin), an attribute of God, is drown from the Brihadaranyaka Upanishad. The epithet, nirvairu of the Parabrahma is a corollary of the doctrine of immanence. The Amritbindupanishad (6) holds that God is impartial. This doctrine differs from the Old Testament concept of ‘chosen people’ or the Quranic concept of the ‘favoured believers’. That the Supreme is a bliss is a Vedantic concept, endorsed both by The Brahmasutra and the Tejabindupanishad. That the supreme is untainted is the doctrine of the Upanishads, later adopted by the Nath samparadaya.

Guru Nanak has sung the glory of Om (G.929-930) as the creator of the Vedas, etc. Needless to say the Om is the Vedic mantra as well as a pan Hindu symbol. Expressions like, ‘Uradh mula jasu sakha’ (Guru Nanak, g.503), ‘Neta neta kathanti beda’ (Guru Arjuna, g.1359), ‘Brahamgiani sada niralepa jaise jalamahi kamal alepa’ (Guru Arjuna, G. 272) are but echoes of the Vedas and the Bhagvadgita.

The Philosophical nomenclature of the Granth viz Parabrahma, Ghata, Pinda, Atama, Moksha, Mukti, jeevan Mukti, Maya, Mithya, Sarguna and Nirguna, Bharamanda, Jogu (yoga), Raja Jogu (rajayoga), Isaru (Ishvara) is same as that of the Sanatana Hindu religion. The term like four yugas, four padaratha goals of life-viz the purushartha, tribhuvana, amrita, lakha chourasiha 84 lakh species, which appear so often in the Grantha denote its Sanatana Hindu ethos.

The rejection by the Sikh Gurus of the Vedic rituals and their insistence on inner realization of God is consistent with the Upanishads. Although the hymns of the Granth repeatedly describe god as formless, a nirguna, they do describe, in many places, the physical attributes of God. Wherever they do so, the description is invariably that of Vishnu or one of His incarnations (G.567, 1082, 1402).

The rejection of idol worship by the Sikh gurus has been mistakenly interpreted as rejection of the Sanatana Hindu religion. ‘He is not in any symbol’ declares the Brahma sutra ((4.1-4). Idol worship is not an essential component of the Sanatana Hindu religion." Note there were idols in the Golden Temple till 1905.

Link to Dr Godbole's piece is at the end of this article. Here are some more insights that could make you question whether Sikh Dharma different from Sanatana Dharma.

One, Nanak started a reformist movement and not a separate religion. Muslim oppression and the Bhakti Movement were responsible for the birth and growth of Sikh Dharma. Modern day Sikhism is a post British construct as we shall read later.

Two, the roots of Sikhism lie deep in the Bhakti form of Hinduism and Vedanta. Nanak picked up some of its salient features – belief in one God who is omnipresent, unity of mankind, emphasis of work as a moral obligation, the gentle way of Sahaj to approach God while fulfilling domestic obligations etc.

Three, Khushwant Singh wrote, "while the Adi-first Granth is essentially a distillation of Vedanta in Punjabi, the last Dasam - tenth is a compilation of tales of valor of Hindu goddesses. Of the 15,028 names of Gods that appear in the Adi Granth, Hari occurs over 8,000 times, Ram 2533 times followed by Prabhu, Gopal, Govind and other Hindu names for the divine. The popular Sikh coinage Wah Guru appears only 16 times."

Four, there are Sindhis (mainly Amils) who follow the Granth Sahib. From the first Muslim invasion in 712 a.d., Sindhis had to bear the brunt of Musilm oppression, torture, destruction of temples, forced conversions. When Nanak visited Sindh, taken in by his spirit of love and affection for the oppressed, they decided to become his followers.

Five, when Shah Shuja, the ex – ruler of Kabul sought Maharaj Ranjit Singh's alliance in 1831, among Ranjit Singh’s demands were the prohibition of cow slaughter throughout Afghanistan and delivery of the gates of Somnath. Would these demands, that were dear to the Hindus, have been made if Ranjit Singh considered Sikhism a different religion?

Six, some argue that the Sikh way for marriage is different from the Punjabi Hindu one. According to W. H. Mcleod, author of Historical Dictionary of Sikhism, “Anand Karaj was not performed until the middle of the 19th century although it is certain that at least the Anand Sahib (q.v.) portion was well established for a long time before that. The marriage ceremony was, however, essentially a Hindu one performed around a sacred fire. The introduction of Anand Karaj was a major concern of the Singh Sabha, an emphatic demonstration that Sikhs were not Hindus.”

Seven, coming to why the first son was made a Sikh? Veena Talwar, author of Dowry Murder, the Imperial Origins of a Culture Crime wrote, “To prevent the kind of mutiny they had experienced from sepoys in 1857, the British organized religiously segregated regimental units from the alleged 'martial races', Sikhs, Pathans, Rajputs and Gurkhas. This severely restricted Hindus of other castes who wanted to join the army, particularly Khatris, who had served in Maharaja Ranjit Singh's forces. It is important to mention that Hindus, particularly Khatris, who were acknowledged as Kshatriyas but were arbitrarily limped together with the 'trading castes' in the British census reports (since large number of them were educated and engaged in trade), were seldom accepted into the British military service. The Khatris, who had also been landholders, acquired further acreage till the Punjab Land Alienation Act of 1900 forbade them to do so as a 'non-agricultural' tribe. Many families got around this artificially imposed caste barrier by raising one or more son as a Sikh, chiefly by having them adopt the name Singh and grow hair/beard to match."

The children of such Sikhs became Sikhs and so. Till a couple of generations ago, the same family had brothers, one Hindu and another Sikh.

Eight, in order to safeguard their geo-political interests the British took various decisions that divided Punjab into Hindu and Sikh. Please read How the British divided Punjab into Hindu and Sikh - link at end of this article.

It is worth noting that:

Maharaja Ranjit Singh donated gold for three temples of which two are called Hindu today. These are Kashi Vishwanath Mandir and Jawalamukhi in Himachal Pradesh besides Golden Temple.

Even today an ancient Shivalaya exists inside the Golden Temple complex.

The Siachen War Memorial has a plaque which quotes Guru Govind Singhji and reads, “Oh Lord Shiva, grant me this boon that, I never shy away from doing good deeds. I should never be frightened away from fighting for Justice, Dharma and Rightful Cause and I should be determined to emerge victorious from this battle. Every soldier to have a pure heart and mind and let not greed come near him. When the time comes for my soul to unite with yours I should die fighting in the battle field.”

Four, the Sikh Crown Prince Krishna Singh from the royal family of Patiala (son of Maharaja Mangal Singh) would have become king if he had not taken sanyas. Instead his eldest step brother Maharaja Karam Singh became king. “After a life full of action and adventure, the prince took sannyas - the vows of a renunciate - in his fiftieth year.  He entered the Udasi sect of sannyasin, assumed the monastic name of Vishnudas” and is now remembered as Tapaswiji. Born in 1770, he gave up his body in 1955. To know more read Tapaswiji, A Biography by T.S. Anantha Murthy.

Five, during a 2012 visit to Naina Devi Temple in Himachal Pradesh the author saw many Sikhs there. Earlier you read that Sikhs were against idol-worship.

The author leaves it to you to decide whether Sikhs are part of the larger Hindu community or not.

This article is based on inputs from The History and Culture of Indian People by the Bhartiya Vidya Bhavan, The Sikhs of Punjab by Grewal, my E book published by Swarajyamag.com

Also read:
1. How the British divided Punjab into Hindu and Sikh
2. The unbreakable Hindu Sikh bond
3. How the British sowed the seeds for the Khalistan Movement before Indians took over
4. History of Sikhs
5. How California Gurudwaras cemented the Hindu Sikh divide
6. Basavanna and Lingayat Movement
7. History of Punjabi