Sikhism as we know it today is the result of the teachings of the ten Gurus, the first of which was Guru Nanak (1469-1539) and the tenth and last of which was Guru Gobind Singh (1666-1708).
Guru Nanak spread a simple message: "We are all one, created by the One Creator of all Creation." There is no definitive biography of Guru Nanak, though there have been many attempts to write the story of his life by his devotees after his death.
According to Dr. Hari Ram Gupta, author of A Life-Sketch of Guru Nanak, Nanak started his mission at a time when both Hinduism and Islam as practiced in the Indian Subcontinent had become distorted and degraded. The caste system was at its worst, and all kinds of corruption had become rampant in society. Men of vision were worried, and they attacked the rot that had set in the society. Rather than address the socio-political problems, the reformers of the day tried to initiate a spiritual movement that would turn people towards God. They believed that this was the way to cure the ills of the society.
Guru Nanak was indeed the most important of these reformers. He was born to a simple Hindu family. From an early age, he made friends with both Hindus and Muslims and acquired a good knowledge of Hinduism and Islam. He used to spend long hours in discussions with Muslim and Hindu holy men of the area.
There is a story of how he disappeared for three days and came back with enlightenment. It is reported that he was no longer the same person he had been. Then he uttered these words:
"There is but One God, His name is Truth, He is the Creator, He fears none, He is without hate, He never dies, He is beyond the cycle of births and death, He is self illuminated, He is realized by the kindness of the True Guru. He was True in the beginning, He was True when the ages commenced and has ever been True, He is also True now." (Japji)
These words are enshrined at the beginning of the Sikh holy scripture, the Guru Granth Sahib. It was 1499 and Guru Nanak was thirty years old at this time.
After this, with a Muslim companion, Guru Nanak undertook long journeys as part of a spiritual mission. He took twelve years to return from this first journey. He then set out on a second journey traveling as far south as Sri Lanka. On his third journey Guru Nanak traveled to the north to Tibet.
Guru Nanak visited Sheikh Ibrahim, the Muslim successor of Baba Farid, the great Sufi dervish of the twelfth century at Ajodhan. When asked by Ibrahim which of the two religions was the true way to attain God, Guru Nanak replied, "If there is one God, then there is only His way to attain Him, not another. One must follow that way and reject the other. Worship not him who is born only to die, but Him Who is eternal and is contained in the whole universe."
On his fourth great journey Guru Nanak dressed in the blue garb of a Muslim pilgrim and traveled to Makkah. He visited Madinah and Baghdad, too.
After having spent a lifetime in traveling abroad and setting up missions, an aged Nanak returned home to Punjab. He settled down at Kartharpur with his family. People came from far and near to hear his hymns and preaching.
After Guru Nanak’s death in September 1539, his Hindu followers thought him to be a Hindu and his Muslim followers thought him to be a Muslim. That is to say, both Muslims and Hindus viewed him from the perspective of their respective faiths.
It was the later disciples of Nanak who gave shape to a new religion, of which Nanak is considered the first Guru. In 1604, Arjan Dev (one of the ten Gurus) compiled the hymns of Guru Nanak along with the compositions of both Hindu and Muslim holy men, like Jaidev, Surdas, Sheikh Farid, and Kabir. The compiled book was enshrined by Arjan in the Golden Temple and was called the Adi Granth.
It was the tenth Guru, Gobind Singh, who organized the community of Sikhs into a khalsa — "a spiritual brotherhood devoted to purity of thought and action." He taught his followers to wear long hair (kesh, denoting saintly appearance), underwear (kachha, denoting self-control), iron bangle (kara, denoting purity in acts), comb (kangha, denoting cleanliness of mind and body), and sword (kirpan, denoting fight for a just cause).
The Sikh scripture called the Adi Granth (called respectfully as Guru Granth Sahib) is considered the Supreme Spiritual Authority and Head of the Sikh religion, rather than any living person. It contains the works of not only the ten Gurus but also the hymns by sufis like Sheikh Farid (1175 - 1265) and Sheikh Bhikan (who died during the early part of Akbar’s reign).
From the foregoing, we understand the following:
Guru Nanak was a religious reformer at best; he was not the founder of any new religion.
Sikhism is the creation of the Gurus, particularly of Guru Gobind Singh, whose compositions and innovations form the content and the framework of the new religion.
The scripture of Sikhism is not any revelation from God but only the compositions of the Gurus as well as those of certain Muslim and Hindu mystics. For this reason, there is no meaning in talking about the authenticity of the book as a Divine Revelation. Because neither the book nor the authors claim it to have been revealed by God.
As Muslims, we can perform da`wah to the Sikhs, chiefly by appealing to their faith in the Oneness of God. We can tell them that Guru Nanak was most probably a Muslim when he died; the available evidence favors that conclusion. Anyway he did not try to replace Islam with a new religion; on the other hand, his utterances point to the fact that he certainly believed Islam to be the True Religion of God.
And Allah knows best.