Can it not be argued that the same thread Guru Nanak rejected was symbolic of the same idea? If in fact a piece of thread cannot make you a true hindu then surely Amrit and the 5 K's alone cannot make you a true sikh, but my question is whether or not they are even needed to be on the path toward God. Now if you fast forward to the time of the Tenth Guru, Guru Gobind Singh Ji, becoming a true Sikh would require one to take amrit and keep the 5 K's, even though amrit in sikhism is symbolic for something that lays within you by the grace of god. Says Nanak, blessed are those who go about wearing such a thread.' So basically, a young Nanak rejected the ritual thread of the Hindu priest and justified it by suggesting that a simple piece of thread cannot make you a true Hindu, it's almost 'for show' instead a true Hindu or any 'good person' in the eyes of god has all the good attributes within them (this is the interpretation I was told from my grandfather). It will not wear out, nor get soiled, nor be burnt, nor lost. Oh priest! if you have such a thread, Do give it to me. When the priest continued to insist that the young Nanak done the string he went into a trance and sang: Let mercy be the cotton, contentment the thread, Continence the knot and truth the twist. When it was time for Nanak to be invested with the twice born thread the “sacred” thread, called the Janeu, he refused to take part in the ritual. Picking up both languages quickly, he surprised his teacher by composing an acrostic on the Persian language. He soon mastered the Vedas and Sanskrit and was enrolled into a madrassa to study Persian and Arabic.

'Guru Nanak from an early age evidenced a questioning and enquiring mind. This is an anecdote from the early life of Guru Nanak Dev Ji For my example, I will cite from 'SikhiWiki' which is a wikipedia like site dedicated to information on sikhism.

I know there's no real contradiction between sikhism and science but I think that there are some contradictions between the teachings, practices and lives of the gurus. Despite the fact I think I 'know' a lot about these relegions, I cant help but waiver between being agnostic and athiest (lol, this has a lot to do with me taking science courses cuz my punjabi family wants me to become a doctor, so typical huh?). Growing up however I started becoming less and less religious although, I think I have a considerable amount of knowledge pretaining to sikhism and other relegions. I come from a gur-sikh family and when I was little my grandpa would read stories to me about the teachings, practices and lives of the Gurus.

Amrit sikhiwiki