Additional reference material such as sounds, costumes, and animation can be found inside the project's code. The whole "Flap is a Christian" thing is a joke that's a big part of the character, but you don't have to include it in the series if you don't want to (you know, because religion and stuff). I can provide insight on how to portray Christianity in the series if you choose to include it. The cross is just an accessory. A costume without it can be found inside the project.
