Medieval attitudes toward Christianity, for lack of better terms, was quite prude. They followed everything in the good book (Bible) to a tee and did not stray from it at all. Their whole lives revolved around the church. When the idea of humanism came around in the Renaissance, it changed everything, because painters, scholars, etc. looked back to antiquity, and recovered the ancient's way of art, etc. This theory focused more on humans in the here and now and living life to the fullest, and they were not necessarily completely concerned about living their lives for God so there is a place for them in heaven when they die.
