Let's look at the question, Is Sikhism an Offshoot of Hinduism, from the perspective of a Muslim from the time of Nanak. This may be a crazy approach, but maybe I can make my conclusion clear a different way. This passage is from Chapter I of The Sikh Religion by Macauliffe. It is the first literary work in which Sikhism is described as a distinct and unique religion.
The Sikh Religion - Volume I
After three days the Guru came forth from the forest. The people thought he had been drowned in the neighbouring river; and how had he returned to life? He then went home, and gave all that he had to the poor. A great crowd assembled, and Nawab Daulat Khan, the Governor, also came. He inquired what had happened to Nanak, but received no reply. Understanding, however, that the Guru's acts were the result of his abandonment of this world, the Governor felt sad, said it was a great pity, and went home.
It was the general belief at this time that Nanak was, possessed with an evil spirit, and a Mulla or Muhammadan priest was summoned to exorcise it. The Mulla began to write an amulet to hang round Nanak's neck. While the Mulla was writing Nanak uttered the following:--
When the field is spoiled where is the harvest heap?
Cursed are the lives of those who write God's name and sell it.
The Mulla, paying no attention to Nanak's serious objurgation, continued the ceremony of exorcism and finally addressed the supposed evil spirit, 'Who art thou?' The following reply issued from Nanak's mouth:--
Some say poor Nanak is a sprite, some say that he is a demon,
Others again that he is a man.
Those who were present then concluded that Nanak was not possessed, but had become insane.
On hearing this Nanak ordered Mardana to play the rebeck and continued the stanza:--
Simpleton Nanak hath become mad upon the Lord.[1]
And knoweth none other than God.
When one is mad with the fear of God,
And recognizeth none other than the one God,
He is known as mad when he doeth this one thing--
When he obeyeth the Master's order--in what else is there wisdom?
When man loveth the Lord and deemeth himself worthless,
And the rest of the world good, he is called mad.[2]
After this, Guru Nanak donned a religious costume and associated constantly with religious men. He remained silent for one day, and the next he uttered the pregnant announcement, 'There is no Hindu and no Musalman.' The Sikhs interpret this to mean generally that both Hindus and Muhammadans had forgotten the precepts of their religions. On a complaint made by the Nawab's Qazi, or expounder of Muhammadan law, the Guru was summoned before Daulat Khan to give an explanation of his words. He refused to go, saying, 'What have I to do with your Khan?' The Guru was again called a madman. His mind was full of his mission, and whenever he spoke be merely said, 'There is no Hindu and no Musalman.' The Qazi was not slow to make another representation to the Governor on the impropriety of Nanak's utterance. Upon this the Governor sent for him. A footman went and told the Guru that the Governor had requested him to come to him. Then Guru Nanak stood up and went to the Governor. The Governor addressed him, 'Nanak, it is my misfortune that such an officer as thou should have become a faqir.' The Governor then seated him beside him, and directed his Qazi to ask, now that Nanak was in conversational mood, the meaning of his utterance. The Qazi became thoughtful, and smiled. He then asked Nanak, 'What hath happened to thee, that thou sayest there is no Hindu and no Musalman?
The Guru, not being engaged in controversy with Hindus at the time, gave no answer to the first part of the question. In explanation of his statement that there was no Musalman he uttered the following:--
To be[1] a Musalman is difficult; if one be really so, then one may be called a Musalman.
Let one first love the religion of saints,[2] and put aside pride and pelf[3] as the file
removeth rust.
Let him accept the religion of his pilots, and dismiss anxiety regarding death or life;[4]
Let him heartily obey the will of God, worship the Creator, and efface himself--
When he is kind to all men, then Nanak, shall he be indeed a Musalman.[5]
The Qazi then put further questions to the Guru. The Guru called on Mardana to play the rebeck, and sang to it the following replies and instructions adapted for Muhammadans:--
Make kindness thy mosque, sincerity thy prayer-carpet, what is just and lawful thy Quran,
Modesty thy circumcision, civility thy fasting, so shalt thou be a Musalman;
Make right conduct thy Kaaba,[6] truth thy spiritual guide, good works thy creed and thy prayer,
The will of God thy rosary, and God will preserve thine honour, O Nanak
Nanak, let others' goods[1] be to thee as swine
to the Musalman and kine
to the Hindu;[2]
Hindu and Musalman spiritual teachers will go bail for thee if thou eat not carrion.[3]
Thou shalt not go to heaven by lip service; it is by the practice of truth thou shalt be delivered.
Unlawful food will not become lawful by putting spices[4] therein.
Nanak, from false words only falsehood can be obtained.
There are five prayers, five times for prayer, and five names for them[5]--
The first should be truth, the second what is right, the third charity in God's name,
The fourth good intentions, the fifth the praise and glory of God.
If thou make good works the creed thou repeatest, thou shalt be a Musalman.
They who are false, O Nanak, shall only obtain what is altogether false.
The Qazi became astonished at being thus lectured. Prayers had become to him a matter of idle lip-repetition of Arabic texts, while his mind was occupied with his worldly affairs.
It was now the time for afternoon prayer. The whole company, including Nanak, went to the mosque. Up rose the Qazi and began the service. The Guru looked towards him and laughed in his face. When prayer was over, the Qazi complained to the Nawab of Nanak's conduct. The Guru said he had laughed because the Qazi's prayer was not accepted of God. The Qazi asked Nanak to state the reason for his conclusion. The Guru replied that immediately before prayer the Qazi had unloosed a new-born filly. While he ostensibly performed divine service, he remembered there was a well in the enclosure, and his mind was filled with apprehension lest the filly should fall into it. His heart was therefore not in his devotions. The Guru informed the Nawab also that while he was pretending to pray, he was thinking of purchasing horses in Kabul. Both admitted the truth of the Guru's statements, said he was favoured of God, and fell at his feet. The Guru then uttered the following:--
He is a Musalman who effaceth himself,
Who maketh truth and contentment his holy creed,
Who neither toucheth what is standing, nor eateth what hath fallen--
Such a Musalman shall go to Paradise.
The whole company of Musalmans at the capital--the descendants of the Prophet, the tribe of shaikhs,[l] the qazi, the muftis,[2] and the Nawab himself, were all amazed at Nanak's words. The Muhammadans then asked the Guru to tell them of the power and authority of his God, and how salvation could be obtained. Upon this the Guru addressed them as follows:--
At God's gate there dwell thousands of Muhammads, thousands of Brahmas, of Vishnus, and of Shivs;[3]
Thousands upon thousands of exalted Rams,[4] thousands of spiritual guides, thousands of religious garbs;
Thousands upon thousands of celibates, true men, and Sanyasis;[1]
Thousands upon thousands of Gorakhs,[2] thousands upon thousands of superiors of Jogis;
Thousands upon thousands of men sitting in attitudes of contemplation, gurus, and their disciples who make supplications;
Thousands upon thousands of goddesses and gods, thousands of demons;
Thousands upon thousands of Muhammadan priests, prophets, spiritual leaders, thousands upon thousands of qazis, mullas, and shaikhs--
None of them obtaineth peace of mind without the instruction of the true guru.
How many hundreds of thousands of sidhs[3] and strivers,[4] yea, countless and endless!
All are impure without meditating on the word of the true guru.
There is one Lord over all spiritual lords, the Creator whose name is true.
Nanak, His worth cannot be ascertained; He is endless and incalculable.[5]
It is said that Daulat Khan, the Musalman ruler, on hearing this sublime hymn, fell at Guru Nanak's feet. The people admitted that God was speaking through Nanak's mouth, and that it was useless to catechize him further. The Nawab, in an outburst of affectionate admiration, offered him a sacrifice of his authority and estate. Nanak, however, was in no need of temporal possessions, and went again into the society of religious men. They too offered him their homage, and averred that he was desirous of the truth and abode in its performance. Nanak replied:--
My beloved, this body, first steeped in the base of worldliness,[1] hath taken the dye of avarice.
My beloved, such robe[2] pleaseth not my Spouse; How can woman thus dressed go to His couch?
I am a sacrifice, O Benign One, I am a sacrifice unto Thee.
I am a sacrifice unto those who repeat Thy name.
Unto those who repeat Thy name I am ever a sacrifice.
Were this body, my beloved friends, to become a dyer's vat, the Name to be put into it as madder,
And the Lord the Dyer to dye therewith, such colour had never been seen.
O my beloved, the Bridegroom is with those whose robes are
thus dyed.
Nanak's prayer is that he may obtain the dust of such persons' feet.
God Himself it is who decketh, it is He who dyeth, it is He who looketh with the eye of favour.
Nanak, if the bride be pleasing to the Bridegroom, he will enjoy her of his own accord.[3]
Upon this the faqirs kissed the Guru's feet, the Governor also came, and all the people, both Hindu and Musalman, attended to salute and take final leave of him. The story continues.