Was Guru Nanak a Hindu because he was born in a Hindu family?

First of all, we all should know that Guru Nanak Sahib Ji never accepted any principle of Hinduism nor Islam. Just because He was born in a Hindu family doesn’t mean that he was a Hindu. Your character makes you a person. If you are born in a Hindu family but you have never done anything like a Hindu and you oppose Hinduism then you wouldn’t be a Hindu. Hindu is he who follows what Hinduism says such as worshiping stones etc. Guru Nanak Ji never ever worshiped stones. This is just one example. Guru Ji always worshiped one almighty God. Guru Ji rejected the idea of wearing Janju (piece of thread) at young age and later didn’t take any rounds around the fire when He married Mata Sulakhni Ji. Guru Ji started a new religion, Sikhism, and preached the message of One God. Bhai Gurdas Ji says:

ਮਾਰਿਆ ਸਿੱਕਾ ਜਗਤ ਵਿਚ ਨਾਨਕ ਨਿਰਮਲ ਪੰਥ ਚਲਾਯਾ॥
“In the world, he established the authority (of his doctrines) and started a religion, devoid of any impurity (nirmal panth).”

Which means Guru Nanak Ji started a new religion. Guru Ji taught Sikhs to believe in one God. Whatever Guru Nanak Ji taught He practiced it first. He did everything first then taught Sikhs to do it in essence practicing what he preached. Guru Ji said, “There is no Hindu, no Muslim” meaning that no one is born Hindu nor a Muslim because what one does makes him Hindu or Muslim. Worshiping stones and believing in more than one God makes you a Hindu; fasting and following what Quran says makes you a Muslim. How can you be a Muslim if you don’t fast, believe in Quran and pray five times a day. How can you be a Hindu if you don’t wear Janju, worship 330 million Gods and believe in castes? Guru Ji taught equality and gave equal rights to women.

This is what Gurbani says about Hinduism:

ਹਿੰਦੂ ਅੰਨ੍ਹ੍ਹਾ ਤੁਰਕੂ ਕਾਣਾ ॥ ਦੁਹਾਂ ਤੇ ਗਿਆਨੀ ਸਿਆਣਾ ॥
“The Hindu is sightless; the Muslim has only one eye. The spiritual teacher is wiser than both of them.” (Ang 875)

ਹਜ ਕਾਬੈ ਜਾਉ ਨ ਤੀਰਥ ਪੂਜਾ ॥ ਏਕੋ ਸੇਵੀ ਅਵਰੁ ਨ ਦੂਜਾ ॥੨॥
“I do not make pilgrimages to Mecca, nor do I worship at Hindu sacred shrines. I serve the One Lord, and not any other.”
(Ang 1136)

ਪੂਜਾ ਕਰਉ ਨ ਨਿਵਾਜ ਗੁਜਾਰਉ ॥
“I do not perform Hindu worship services, nor do I offer the Muslim prayers.”
(Ang 1136)

ਨਾ ਹਮ ਹਿੰਦੂ ਨ ਮੁਸਲਮਾਨ ॥
“I am not a Hindu, nor am I a Muslim.”
(Ang 1136)

ਪੰਡਿਤ ਮੁਲਾਂ ਛਾਡੇ ਦੋਊ ॥੧॥ ਰਹਾਉ ॥
“I have abandoned both the Pandits, the Hindu religious scholars, and the Mullahs, the Muslim priests.”
(Ang 1159)