Seek the devil, and you shall find it.
Similar to the theme of crime and punishment. Unless a person has no conscience, any wrongdoing will devour every joy a person has, little by little.
The Christian symbol was shown multiple times in the film, it's the only moral compass of good and evil, the world is too random, our pain is too deep, without that symbol, our action will always be justified by "feelings". The way to hell is paved with good intentions. Great film, great performance, and great story! Loved it!