Let me rephrase:
What makes you stick to your moral system? And what makes it less arbitrary than the reason that an atheist, say, sticks to his moral system?

The difference, in that case, is only superficial. The religious person decides on the principle "the morals religion X dictates are now aslso mine," and therefore gets a bundle of morals at a time, while the secular person chooses them individually (ideally). It's just the difference between buying products seperately and getting them at a discount bundle.