morals and ethics are always vaguely defined. However Shawn’s definition quite fit in.

Moral, as well as ethics, are rules we make because we like them. So the
answer of how religious people decide what’s moral is simple. Do they
like it?

Monogamy, for example. Do you like it? Not if you’re stud. But if you’re single
and don’t get any, you will like it like hell and “somehow” it’ll become morality. Somehow you will think and try to convince everyone that it’s what God’s want.

In other word, it’s just like every one else. The difference is atheists can explain the benefit to people around them. The religion uses more absurd pretexts