Tauriq Moosa, an ethicist who teaches critical thinking at the university of Cape Town, has an excellent article on why it makes no sense to defend morality based on religion: The Flaws in Defending Morality With Religion. I recommend his article because it is so clearly written and logically destroys any argument for basing moral "decisions" on religion. He destroys the 3 typical arguments put forth by believers:

1) "Something is moral because god says it is." This does not work because it means something is good or bad depending on the whims of a being who is not only not us, but different than us, and that we cannot understand. It's also stupid because for example, the Bible approves of slavery, infanticide, and trophy wives, and no Christian these days would support these practices.

2) "God would only command something if it's right." This one does not take long, it's a simple tautology: "god says it's good because it's good." A variant of this is that god is inherently good therefore he could never do command us to do evil. This does not help us either since it is circular reasoning; we haven't even begun discussing the meaning of "good"!

Moosa says that defending morality with religion is flawed even if the religious person happens to side with non-believers; for example, a Christian who supports homosexual rights because she is a "good Christian" is just as wrong as a Christian fundamentalist who opposes gay rights based on the Bible. They both think their morality is right because it's based on their religion, and because of that, they are both wrong, even if, of course, we prefer the Christian who defends gay rights. The nutshell argument is that to defend a moral decision, you need to be able to engage freely, to reason morally, if god hands you down a commandment, you're obeying, you're doing what someone else tells you to do, not making a moral decision. If you are obeying a human authority, you're still not making a moral decision but you can engage the person being the moral authority. You cannot engage a god who either does not exist or if it did, it is unknowable, unreachable, etc. Doing something because god or a sacred book tells you it's good, means that you are making any moral decisions. This a fundamental flaw with religion-based ethics, whether we agree with their conclusions or not.