The moral law exists and we did not give it to ourselves. But there cannot be law without a law-giver, and the only possible source of this law is God. I.e., if there is no God, then there is no moral law. But there is such a law. Therefore there must be a God.
All people believe, at some level, in such a thing as the moral law.
Hence, this absolute moral law which we find in our minds, and which comes to us from outside ourselves, must originate in an absolute mind. Rashdall observes that people do not doubt the existence of an objective reality existing independently of themselves just because their sense perceptions of it vary. The success of the moral law argument depends on whether there is such a thing as an objective moral order transcending our perceptions, inclinations and tastes. Many argue that there is not, citing the variety of sentiments and practices that exists from person to person and culture to culture. So the presence of differences of opinion in no way refutes the possibility of an objectively existing moral order.