Romans 12:20

Instead of doing one’s enemy an unkindness the believer should do him or her positive good (cf. Matthew 5:44). This may result in the antagonist feeling ashamed, acknowledging his error, and even turning to God in repentance.

One interpretation of heaping burning coals on his head is that it figuratively describes doing good that results in the conviction and shame of the enemy. The expression supposedly alludes to the old custom of carrying burning coals in a pan. When one’s fire went out at home, a person would have to go to a neighbor and request hot coals that he or she would then carry home in a pan, typically on the head. Carrying the coals involved some danger, discomfort, and uneasiness for the person carrying them. Nevertheless they were the evidence of the neighbor’s love. Likewise the person who receives good for evil feels uncomfortable because of his neighbor’s love. This guilt may convict the wrongdoer of his or her ways in a gentle manner. [Note: Cf. Griffith Thomas, St. Paul’s Epistle . . ., p. 347.]

A better interpretation, I think, takes the burning coals as a figure of God’s judgment that will come on the enemy if he persists in his antagonism. The figure of "coals of fire" in the Old Testament consistently refers to God’s anger and judgment (cf. 2 Samuel 22:9; 2 Samuel 22:13; Psalms 11:6; Psalms 18:13; Psalms 140:9-10; Proverbs 25:21-22). Thus the meaning appears to be that the Christian can return good for evil with the assurance that God will eventually punish his or her enemy. [Note: See John N. Day, "’Coals of Fire’ in Romans 12:19-20," Bibliotheca Sacra 160:640 (October-December 2003):414-20; John Piper, "Love Your Enemies": Jesus’ Love Command in the Synoptic Gospels and in the Early Christian Paraenesis, p. 115; and Krister Stendahl, "Hate, Non-Retaliation, and Love: 1 QS x, 17-20 and Romans 12:19-20," Harvard Theological Review 55(1962):352. See Witmer, p. 490, for a third view.]