Divine command theory holds that what makes an action obligatory is that God commands it, and what makes an action wrong is that God forbids it. Morality on this account is grounded in divine will rather than in a standard independent of it.
Its classic difficulty is the Euthyphro dilemma, posed in Plato's dialogue: is the pious loved by the gods because it is pious, or is it pious because it is loved by the gods? Applied to monotheism, the question becomes whether God commands what is good because it is good, or whether it is good because God commands it. The first answer appears to subordinate God to a standard God did not make. The second appears to make morality arbitrary, since if God had commanded cruelty then cruelty would have been right, a consequence most theists find unacceptable.
The standard reply, developed most fully by Robert Adams, is a modified divine command theory: obligations derive from the commands of a God who is by nature loving and just, so the commands cannot be arbitrary because God's nature is not arbitrary, and God's nature rather than God's bare will is the standard. Critics ask whether this relocates the dilemma rather than resolving it, since one may then ask whether God's nature is good by some further measure. Defenders answer that the question terminates there, because God's nature is what goodness is rather than something that conforms to it.
Theological traditions divide along the same line. Voluntarism, emphasising divine will, is associated with Duns Scotus and William of Ockham in Christian scholasticism and with Ash'ari theology in Islam, which held that reason cannot determine good and evil independently of revelation. Intellectualism, holding that God commands what is good because of what it is, is associated with Aquinas and with natural law, and in Islam with the Mu'tazila, who held that good and evil are knowable by reason and that God commands accordingly.
Jewish tradition contains both emphases, and the binding of Isaac in Genesis 22 is where the question is posed most sharply. Kierkegaard's Fear and Trembling reads it as a "teleological suspension of the ethical", in which the demand of faith overrides the moral law. Much rabbinic and modern Jewish commentary resists that reading, and Abraham's argument with God over Sodom, "shall not the judge of all the earth do right?", is cited on the other side as showing a patriarch holding God to a standard.
The debate has practical stakes. If morality is available to reason independently of revelation, then believers and unbelievers can reason together about ethics, and religious traditions can be criticised on moral grounds from outside. If it is not, then moral disagreement across traditions is at bottom a disagreement about which revelation is authentic, and there is no common court. Most contemporary religious ethicists hold some version of the first position, and much interreligious ethical work depends on it.