The Euthyphro dilemma: either God commands things because they are good, so goodness is independent of him, or things are good because he commands them, so morality is arbitrary. Either way God doesn’t ground morality.

This is the classic objection, and every philosophy student will raise it. The standard theistic reply is that the dilemma is a false one: goodness is neither above God nor arbitrary but is God’s own nature, so his commands express what he is rather than what he happens to decide. Whether that reply works is debated (critics ask what makes God’s nature good), but it is the reply, and it has a long pedigree from Augustine to Robert Adams. The argument here points to a transcendent source, not automatically to the Christian God, and it leaves room for exactly this debate.