Humanism is “the rejection of religion in favor of a belief in the advancement of humanity by its own efforts.” That is the definition, I find in one of my dictionaries.
Humanists believe that they have it in them to achieve, to make a success. They are self-reliant and they are self-confident.
Man as he is by nature does not believe in a god. He does not need a god. He is his own god. He is autonomous, self-centred and independent. (He forgets that only the God of the Bible is self-existent and that he is a dependent being!)
The humanist and the man as he is by nature thinks he can save himself.
He rejects religion, because religion tells him that he cannot save himself.