There are two ways 'christianity' could be adapted into film form, literally (films based on religious text and events) and moralistically (films based on christian morals and themes).

In literal interpretations, the bible could make for fantastic movies, in a similar way that greek mythology has lended itself to great stories. The problem is, While regular viewers want an interesting film, religious viewers want a direct reproduction of the religious text, basically a movie that preaches at people. Typically direct adaptions make for horribly boring and preachy films, inspired work leaves religious folk screaming foul, and anything inbetween just makes everyone unhappy (we say this with the recent Noah movie).

The other path is moralistic adaptions, making movies inspired by christian themes and morals; This is nice and all until you ask what christian morals actually are. Pretty much every sect slightly disagrees on the morals of the bible; and at its core, the morality of christianity isn't about being a good person or whatnot, but about worshipping this very specific deity, everything else is secondary. So making a movie where the moral/theme revolves around worshipping this very specific god, again we get the issue of a preachy film that only the very religious will seek.