What are some of the differences between ancient (especially greek and roman) and medieval art? what are some of the cultural changes that account for those differences?

Greco-Roman art has an emphasis on Humanism and the perfection of ones body, along with pagan characters. In medieval art, many of the paintings were of Biblical scenes. Many of the Greco-Roman age sculptures were naked, and in medieval times it was sometimes seen as heresy/sodomy to have naked, youthful boys as the subject, thus being condemned by the Church.