Before Christianity there were gods and goddesses and a multitudes of deities that people worshiped. In their tribute to these deities we have art and literature. The worship of the earth is now considered a pagan thing. But back to the beginnings of civilization this was religion and beautiful art was created, so when Christianity became a super religion one or the other had to give, however in art there was a union.
An example of the assimilation of pagan symbols and Christianity is the sun and God as being one in the same. Before Christianity became super it was the underdog but thanks to Constantine the further prosecution of Christianity was eradicated with the Edict of Milan.
"The Early Christian Art employed 'Symbolism,' for example the fish, represented Christ; lamb, Jesus' sacrifice; lambs, Christian sacrifice; cross; Jesus' crucifixion; anchor, hope & steadfastness; garden, paradise; bird, soul; dove, peace & purity; and so on"(www.ezinearticles.com/?Early-Christian-Art). The same Artist were now creating art in a christian context drew from the art and styles that were so familiar to them.
Many of the Greek gods reelected religious christian characters. Such as Apollo and the early beardless Jesus. The mixing of these two styles is very interesting although when it come to the blending of these two elements the christian influence is the dominant however the pagan symbolism is always subtly present.
Christian art simply adapted to styles that had already been wrought, they just transformed people and stories to fit with their own