The moral argument appeals to the existence of moral laws as evidence of God’s existence. According to this argument, there couldn’t be such a thing as morality without God; to use the words that Sartre attributed to Dostoyevsky, “If there is no God, then everything is permissible.” That there are moral laws, then, that not everything is permissible, proves that God exists. Some facts are facts about the way that the world is. It is a fact that cats eat mice because there are lots of animals out there, cats, and lots of them eat mice. It is a fact that Paris is the capital of France because there exists a city called Paris that is the capital of France. For most facts, there are objects in the world that make them true.

Morality Consists of a Set of Commands
Moral facts aren’t like that. The fact that we ought to do something about the problem of famine isn’t a fact about the way that the world is, it’s a fact about the way that the world ought to be. There is nothing out there in the physical world that makes moral facts true.
This is because moral facts aren’t descriptive, they’re prescriptive; moral facts have the form of commands.

Commands Imply a Commander
There are some things that can’t exist unless something else exists along with them. There can’t be something that is being carried unless there is something else that is carrying it. There can’t be something that is popular unless there are lots of people that like it.
Commands are like this; commands can’t exist without something else existing that commanded them.
The moral argument seeks to exploit this fact; If moral facts are a kind a command, the moral argument asks, then who commanded morality? To answer this question, the moral argument suggests that we look at the importance of morality.

Morality is Ultimately Authoritative
Morality is of over-riding importance. If someone morally ought to do something, then this over-rules any other consideration that might come into play. It might be in my best interests not to give any money to charity, but morally I ought to, so all things considered I ought to. It might be in my best interests to pretend that I’m too busy to see my in-laws on Wednesday so that I can watch the game, but morally I ought not, so all things considered I ought not.
If someone has one reason to do one thing, but morally ought to do another thing, then all things considered they ought to do the other thing. Morality over-rules everything. Morality has ultimate authority.

Ultimately Authoritative Commands Imply an Ultimately Authoritative Commander
Commands, though, are only as authoritative as the person that commands them. If I were to command everyone to pay extra tax so that we could spend more money on the police force, then no one would have to do so. I just don’t have the authority to issue that command. If the government were to command everyone to pay extra tax so that we could spend more money on the police force, though, then that would be different, because it does have that authority.
As morality has more authority than any human person or institution, the moral argument suggests, morality can’t have been commanded by any human person or institution. As morality has ultimate authority, as morality over-rules everything, morality must have been commanded by someone who has authority over everything. The existence of morality thus points us to a being that is greater than any of us and that rules over all creation.
What the Moral Argument Proves
If the moral argument can be defended against the various objections that have been raised against it, then it proves the existence of an author of morality, of a being that has authority over and that actively rules over all creation. Together with the ontological argument, the first cause argument, and the argument from design, this would give us proof that there is a perfect, necessary, and eternal being that created the universe with life in mind and has the authority to tell us how we are to run it. The correct response to this would be to seek God’s will and to practice it.

http://www.existence-of-god.com/moral-argument.html