Found in Plato's dialogue, the modified version is this: Is what is morally good commanded by God because it is morally good, or is it morally good because it is commanded by God?
(1) If divine command theory is true then either (i) morally good acts are willed by God because they are morally good, or (ii) morally good acts are morally good because they are willed by God
(2) If (i) morally good acts are willed by God because they are morally good, then they are morally good independent of God's will
(3) It is not the case that morally good acts are morally good independent of God's will
Therefore:
(4) It is not the case that (i) morally good acts are willed by God because they are morally good
(5) If (ii) morally good acts are morally good because they are willed by God, then there is no reason either to care about God's moral goodness or to worship him
(6) There are reasons both to care about God's moral goodness and to worship him
Therefore:
(7) It is not the case that (ii) morally good acts are morally good because they are willed by God
Therefore:
(8) Divine command theory is false