Because he rose to the haven from the river and after this nanak really did believe in god and teach people about him.
What did God say to Guru Nanak ?
1469 ( but technically Guru Nanak is not a God )
Guru Nanak Dev Ji was blessed by God and was the founder of Sikhism we do not know if Guru Nanak Dev Ji was a scientist
Guru nanak had many people who chose to follow guru nanak on his peaceful path to god and to show that Muslims and hindus are the same tyhus he crreated Sikh (like me)
self realisation
Guru Nanak was remebered for going undeer the water at his local river for 3 days an experiencing god :)
guru nanak dev ji respect all the religion. and he always says god is one and his teches are very nice.
By Remembering God at all times.
Sikhs DO NOT worship Guru Nanak. They worship the Divine Light of God that is in Guru Nanak. Guru Nanak is NOT a deity in the Sikh religion, he is a teacher. Sikhs still bow today to Guru Granth Sahib. That does not mean they worship it, it means they revere the Divine Knowledge that it contains.
Guru Nanak is not a Sikh god, Sikhs believe in one god. The Gurus are 1/2 god, 1/2 human
Guru Nanak spread the word of Sikhism by travelling around the world on foot, getting followers on the way and spreading the word of God.
Guru Nanak Dev Ji has written a lot of bani (hymns) about the praise of God. Most popular bani includes Japji Saheb.