I believe that all religions have basically the same layout. That is we are all products of creation and we need to give a power higher than us the glory and reverence. Pagans believe that everything has a soul and very much polytheistic. Christianity believes there is one God, Jesus was the Savior of our souls and we should believe in him through faith. Pagans and Christians alike believe in their entities through faith. The hills, sun, and moon can be seen (just as Christs' body was seen) but to believe in the soul to pray and give reverence takes real faith.
When Christianity was becoming popular, the vast majority of people were pagans. They prayed to different gods. Constantine ushered in Christianity as the official religion of the state. Christians were converts from the pagan religion. These new Christians were the artists of the day. I believe they held on to some of the pagan influence because it was comfortable. Making these halos around Mary and Jesus' heads it echoed sun worship. The personifications of seasons or ideas was holding on to the idea that everything has a soul (pagan). The advantage of this pagan influence is that it opened up a visual dialogue between the two religions. The Christians however has only one higher God and was refusing to call the men of importance the supreme authority. Propaganda was created by blending the ideas of these two religions in art also. The christian temples had so much artwork that the shear amount of artwork alienated and dominated the culture. The work pushed the pagans further away and drew the christians closer in. So the disadvantage would be basically just telling people what they will believe and making it hard to exist comfortably without being exposed to the opposite day in and day out. They did a kind of constructive eviction.