

Apparently, you get the Devil.

First off, the Bible does not tell you what the Devil looks like, and neither does the Catholic Church. I think a lot of times the reason people reject the belief in something like the Devil is because when we were kids we saw him in Bugs Bunny, and we assume that how he is portrayed in Bugs Bunny is how he actually looks and what Christians believe in. We know that he is a fallen angel, and that he was called "Lucifer", meaning "Bearer of light". In the Bible he is portrayed as a talking serpent at one point and as a dragon at another point. It's possible that before he was made to crawl on his belly, the serpent was a dragon. Whatever the case, I don't think anyone believes that the Devil is literally a talking snake. People who think he has goat legs probably don't know much about Christianity either!

When Christianity spread to pagan places, it was not unusual for them to incorporate pagan ideas to better illustrate their points. So Paul told the people in Athens that the unknown god they worshipped was actually the true God, and Christmas was originally the celebration of the Winter Solstice, and was made Christian when we said "You celebrate that light is increasing in the world (Days are getting longer), well the light of the world is Jesus, so let's celebrate his birth."
So pagan symbols would become associated with the Devil, in some cases the pagan Gods would even be seen as demons. Jesus himself seems to have started the trend by referring to Satan as Beelzebub (Matt 12:25-28), who was actually a Philistine god. Interestingly, though, portraying Satan as being like Pan or a Faun dates back only to the 19th century, when Neo-Paganism was all the rage.
All of this begs a question, though. Most people will have no difficulty accepting that our view of what Satan looks like is probably inaccurate and informed by paganism. But I wonder if the same thing can be extended to our idea of Heaven, or of Hell? When we picture Heaven, are we picturing the same thing that Jesus pictured? I suspect that it would be a fascinating study to try to figure out what Jesus, and the Church, actually means by those words, and how much of it is just our presupposed notions.