Of course, in the modern world we disagree with specific moral rules and no longer practice some of them, such as the rules about slavery and divorce. Indeed, fewer than half of the Ten Commandments are encoded in United States law. But as a theological claim, if God is good, then the moral rules He proclaimed must be good. And, since God is by definition perfect, then the morality He proclaimed must also be perfectly good.
From this viewpoint, it is interesting to ask this question: Is God's morality good because He commanded it, or did He command it because it was good? This is a difficult question, and different forms of Christianity have answered in different ways. It is so difficult that many forms of Christianity have refused to address it. It is a conundrum for all monotheistic religions, including Judaism and Islam.
The conundrum is this: while all Christian and monotheistic believers happily affirm that God and his ethics are good, the possible answers to the question require the affirmation of a second point, and that point is less willingly accepted. There are two possible points, one for each answer to the question, and both are uncomfortable for monotheists.
If God's morality is good because He commanded it, then that means that whatever He commanded would have been equally good. He could have commanded anything and it would have been just as good. God could have decreed that Wednesday, instead of the Sabbath, was the holy day. And that would be good. He could have decreed that murder or theft were good.
Our ethical and moral sense, therefore, comes from God's commands. If he had commanded something else, then Christian moral sensibilities would be different. It is rather uncomfortable to think that Christian morality was open to all possibilities before God uttered His commands, and that He arbitrarily chose to declare some actions good and some actions evil.
The alternative answer to our question resolves this problem, but only by creating another one. If God commanded Christian morality because it was good, that means that each rule in it has an essence of goodness. Due to its inherent nature, then, and not because God said it, each command is good in and of itself. When all moral rules are taken together, that means there is a standard of goodness that is independent of God. The standard did not come from God, because then it would evidence the problem of arbitrariness and actually be the answer discussed above. Instead, this moral standard exists apart from God, and existed before God commanded the Jewish and Christian moral rules.
The problem this causes for Christianity, indeed for any monotheism, is that it creates something ultimate that is not God. It also implies that God is not omnipotent in the area of morality, but consults the standard to ensure the goodness of his moral rules. To be sure, the goodness standard is not a second god, and so does not require the conclusion of polytheism. But it does mean that God is not alone and that He did not create goodness, but instead followed a pre-existing standard of inherent good.
Of course, this theological conundrum has no impact on the specific character of Christianity's moral rules. Its ethical demands remain the same whichever answer one takes, and even if one chooses not to address the question. For in the end, Christianity believes, God's morality requires obedience, not understanding.
Thanks to James Rachel's _The Elements of Moral Philosophy_ (Philadelphia: Temple University Press, 1986). For information, see the section on Divine Command Theory.