"Are we to suppose that no one knew that murder was wrong until the Israelites got to Sinai and got the Ten Commandments? Of course they knew it was wrong, and what that shows is that you don't get your morality from the Bible." That's the assertion I have heard, but what is the answer? Well, to a large extent, I agree!
The Christian claim is not that people need the Bible to know what is right and wrong, nor is the claim that people need to believe in God to know what is right and wrong. The claim is that God must exist for there to be right and wrong, because if God doesn't exist then there is no objective standard, and there is no law that is binding upon us other than what we make up ourselves.
The Bible tells us that we have moral intuitions written upon our hearts by God so that we know right from wrong (Romans 2 v 15), and thus when we sin we sin "with knowledge" (the meaning of the word "conscience"). People knew about sin long before the Bible was written (e.g. Genesis 39 v 9) - the demands of God are something He puts on the hard drive of every person, and furthermore, we have knowledge of the fact that there is judgment for our sins (Romans 1 v 32).
So why do we need the Bible then? A couple of thoughts here, firstly, the Bible tells us a whole lot more than the fact that we have sinned. It tells us about the God we have sinned against, and how an answer has been provided to that problem through the death and resurrection of the Lord Jesus Christ. Second, we can stifle and suppress the voice of conscience, and the Bible gives a written revelation of God's standard.
The Israelites of course knew that murder was wrong, but at Sinai the twelve tribes were constituted a nation, and every nation has a law. Is there a country in the world that doesn't have a law against murder? The reason is so that people know they will be punished by the Government if they do it. At Sinai God gave the law for the nation of Israel. There would be penalties to be faced for violations of those commandments.
So it is with us, God has written the work of the law on our hearts, and there is a penalty to be paid for violating that law (Romans 2 v 12). We are all guilty (Romans 3 v 19), and that is why we all need the answer God has provided for our guilt - His Son (Romans 5 v 1-11).
