People turned away from the church and started to focus more on life rather than life after death because of increasing knowledge in science. And when the printing press came around the ideas really started to spread. Humanism was every where from art, to travellers.
Humanism is an importance to humans rather than divine or supernatural powers. Humanists stress the goodness of human beings and seek ways of solving human problems. It was so popular during the Renaissance because it was such a successful period that they had time to focus on other things like humanism.