God's Will Decides Morality

What is done in accordance with God's will is the best of all things even if it seems to be bad. What is done contrary to God's will and decree is the worst and most unlawful of things--even if men judge that it is very good. Suppose someone slays another in accordance with God's will. This slaying is better than any loving-kindness. Let someone spare another and show him great love and kindness against God's decree. To spare the other's life would be more unholy than any slaying. For it is God's will and not the nature of things that makes the same actions good or bad. Fathers of the Church (vol. 68), St. John Chrysostom, Discourses Against Judaizing Christians, translated by Paul W. Harkins, Discourse IV, section I, paragraph 6 (p. 74)

Circumcision's Relation to Baptism - Paul W. Harkins regarding Chrysostom and Aphrahat

But someone might say: "Is there so much harm in circumcision that it makes Christ's whole plan of redemption[fn9] useless?" Yes, the harm of circumcision is as great as that, not because of its own nature but because of your obstinacy. FN9: Redemption is based on Christ's Incarnation; we participate in it by baptism, not circumcision. Cf. ACW 31.161-62, 232, 328. To accept circumcision is to embrace the Old Law in its entirety; but God has rejected the Old Covenant, and what God has rejected can no longer be good or useful because it is now contrary to his will. Aphrahat, the first great father of the Iranian Church and a contemporary of Chrysostom, also maintained that circumcision never had any salvific value except where combined with faith, and Israel was unfaithful, as the prophets themselves contended. See J. Neusner, "The Jewish-Christian Argument in Fourth-Century Iran: Aphrahat on Circumcision, the Sabbath, and the Dietary Laws," Journal of Ecumenical Studies 7 (1970) 282-90. The Fathers of the Church, vol. 68, St. John Chrysostom, Discourses Against Judaizing Christians, translated by Paul W. Harkins, Discourse II, section (6), p. 37.