John does not turn back because he feels that his spirit will not let him. He has come to the river on his big quest and he feels that he absolutely has to go and see the Place of the Gods, even if it kills him to do so. John says that
I knew that I should have to cross the river and walk in the places of the gods, although the gods ate me up.
Basically, John is pushed on by his faith and his ambition. He believes that he has been called by the gods to go to this place. He also wants to take his place as a great priest. He knows that he has to follow what the gods and his spirit tell him because
It is better to lose one's life than one's spirit, if one is a priest and the son of a priest.
John is terrified of what might happen to him, but he is driven by faith and ambition and he cannot bring himself to give up.