What is the point of morality if you're a theist?

[Esquilax]

If something is from God then it is good if something is against God it is human sin.

So in your view, christianity doesn't have morality at all? It just has edicts?

It's just the nature of what the reality of God is and reality of human freedom of will. There is good and there is evil, right and wrong. We don't invent it we understand it.

Except I'm asking about the criteria of selection for morals, here. Did god reason out the rule against killing, for instance, because the action is directly harmful here on earth, or did he just do it because he doesn't like killing?

Not only have you still not answered my question, you've now put yourself in the position of accepting anything god says as moral regardless of whether or not that thing was previously immoral. If god came to you today and commanded you to murder a child, would you do it?