Morality and the Athiest's Response

In your video entitled "Where do atheists get their morals from ?" you answer that question by saying that all societies are under, what you call, a social contract. Then at the very end of the video you say that this social contract is, in essence, the golden rule, "do unto others as you would want done unto you". So, are we to assume that all societies moralities are based off the Bible's Golden Rule? If not, then what is the difference?
Atheist Answer: 

Do a little research on the Golden Rule to see just how exclusively Biblical it is. Not.

The Bible has plenty of instructions that everyone would agree are great ideas, like "thou shalt not kill". It also has plenty of instructions everyone would agree are terrible ideas, like much of Leviticus. The good ideas are not only common sense, but they can generally be found in other sources pre-dating the Bible by centuries.

Consider the possibility that the good ethical advice in the Bible is not good ethical advice because it's in the Bible, but rather its human authors put it into the Bible because it's good ethical advice. It's a reflection of human nature, much of which is benevolent.

This may make the Bible a good moral guide if you cherry-pick the sensible parts, but it certainly doesn't establish its God as the only source of morals.

- SmartLX