Simple self interest.
If I go around murdering other people then what's to stop other people trying to murder me. It's in my interest to treat people well, just as it's in their interest to treat me well.
It's called the golden rule, and it's far simpler and easier to follow than an abstract and arbitrary set of rules set out by a handful of priests.
I assume that you are a christian, in which case I must ask, do you follow the biblical instruction to not boil a kid in its mother's milk? How about the requirement to stone to death a rape victim who doesn't scream loudly enough?
If not then you aren't following the moral code laid out by your god.
And how about the ratio of religious to non religious prison inmates? If the religious are more moral then surely there should be a greater proportion of non religious people in prison than in the general populace. In fact the reverse is true, the proportion of non religious inmates is far lower than the proportion in the general populace.