The Euthyphro Dilemma
The Euthyphro Dilemma is an ancient problem first postulated by Plato. It challenges religious accounts of morality by asking 'Is an act good because God approves of it, or does God approve of an act because it is good?'.
If the former option is chosen, morality becomes wholly arbitrary. God could descend from the clouds and declare the murder of black people to be morally good, and it would be so. Many theists understandably find this repulsive. Some have argued for this position, but claim that God would never do such a thing, raising further problems with regards to the omnipotence paradox.
If the latter option is chosen, God becomes wholly arbitrary, as he is reduced to merely applauding or booing human actions as they occur, which theists also find unsatisfactory. Again, the omnipotence paradox comes into play.
The Euthyphro Dilemma has stubbornly resisted resolution and remains a serious problem to this day for attempts to integrate theology with ethics. Kant attempted a unique resolution with the moral argument, but was ultimately unsuccessful.