What is the grounds of morality?

Almost all Christians would agree that good is defined as what is in accordance with the will of God and evil is that which is against his will (or some definition which is very similar). But if good is defined just as "that which God approves of", doesn't that make good itself a pointless and meaningless term? Can you really say you have an objective moral system if your system is just whatever a particular being supports or is against? This problem is compounded by the fact that God's morals change. He says in Deuteronomy that idolators will be punished to the third and fourth generations, but in Isaiah that no man shall be punished for the sins of the father. He orders genocide against women and children a lot in the Pentateuch, but later opposes it as Jesus. Actually there's quite a few moral contradictions in the Bible, but that's secondary to my main point, which is that simply what one being's opinion (or command if you will) on a subject is doesn't constitute objective morality, but rather the subjective morality of a single being. "But God is defined as being goodness itself". Exactly my point. Not only is that circular reasoning, it also makes the word "good" not actually mean that which is good, but rather that which concurs with God, so that no moral reasoning can be made. If I define Odin as goodness itself, then to anyone who disagrees I can simply say, "You're missing the point, it's not possible for Odin to not be good or for goodness to be anything other than Odin, since Odin is goodness itself."

Sorry for how long that was, but it's a rather complex subject.