When Christianity was new, pagans saw it as mysterious and secretive. They thought it was like some wacky new cult with some weird rituals! At one time, they thought that the Christians worshiped donkeys! The misconception might have sprung up from another misconception that the Jews were lead to “the promised land” by a donkey! Another thing they thought was that Christians were eating their deity (in a way they are, symbolically). That came up from the whole story about Jesus talking about people eating his flesh and blood which turned into wine. Some ritual was taken to be incest too! Not to mention the whole story of Jesus getting crucified… The pagans thought, “what religion makes their god’s humiliation the central part of the faith?”. It seemed like they were worshiping a condemned criminal. Part of the misconceptions may be because early Christians were persecuted, so they had to practice in secret, and because they were so secretive, they were viewed with suspicion feeding off each other in a big nasty cycle!
(It says “Alexamenos Worships his God” in a mocking tone. Note the donkey-head!)