I'm currently reading Our Sacred Signs by Ori Z. Soltes, and in Chapter Three he discusses the origins and development of (western) Christian art. It's interesting to read how these early Christ-followers in Europe combined their new understanding of Jesus with imagery from their pagan background in ways that were either complementary with their faith, and/or by infusing pagan motifs with new Christian meanings.
One of the most interesting examples he gives is the 4th century sarcophagus of a Bishop Liberius, which depicts Jesus flanked by apostles. Unlike our modern version of Jesus, this depiction shows him as beardless. Although I'm not yet convinced of Soltes' scholarly insights, he offers a possible explanation for a clean-shaven Jesus: the image of Christ as God is based on contemporaneous depictions of “glorious young Greco-Roman Gods” (such as Apollo) who were, apparently, also depicted sans beards. Also, Soltes writes, Pan was portrayed with a beard, and it is this creature which was being used as an inspiration for depictions of Satan and his minions. Soltes frustratingly offers no evidence or images for this assertion (which is the one of the big problems I have so far with his book), so I have no idea whether he's right or not. The whole topic of the origins of early Christian imagery is something I'd like to read more about, but at this point I'm unable to evaluate his ideas.
But, I suspect that as increasing numbers of former pagans joined the ranks of Christianity over the centuries (especially after Constantine made it the state religion of Rome), there was a lot of intermingling of visual elements and ideas as the Church gradually developed its own visual iconography.