It's often just assumed that God-given morality is a coherent idea, leading to questions about morality in the absence of God. Yet there are problems with this conception of morality, mainly that divine command theory makes for an arbitrary set of moral principles. Is murder wrong because God commanded it, or did God command it because it is wrong? If the former, then it's an arbitrary requirement. If the latter, then the principles are external to God. This is the Euthyphro problem.
"But God commands against murder, and you agree murder is wrong. So how can the command be arbitrary?" That's a case of projecting morality onto God, not the other way around. that a God commands us not to murder doesn't make it any less arbitrary as a grounding for that moral precept, any more than God commanding us to murder being moral. Because it would be just what God says without reason, and if there's reason then it's not grounded in God.