It is a well-known story in Sikh history that Guru Nanak disappeared into the river for three days and he was thought to have died.  However, this was actually the time where he experienced union with the Creator.

When Guru Nanak emerged from his 3-day experience of Samadhi (union with the Divine) in the river, his first words were, “there is no Hindu, there is no Muslim”. From that simple statement Guru Nanak declared the belief that there is only one God – then it follows that since there can only be one God (Creator of the universe), then people of all faiths actually worship the same embodiment of the Divine.

Guru Nanak extended this doctrine to include the idea that, if all people worship the same God, then all people are equal. Ideally, this also applies to all people, regardless of caste, color, gender, etc.

It flows from this belief that if all people are equal, then all paths to God, all ways of worship are also equal. Sikhs do not believe that their way is the only way and, therefore, Sikhs do not proselytize their ideology or beliefs.

~ From Sikh Dharma International – https://www.sikhdharma.org/ideology-beliefs/