12:17-21 Since men became enemies to God, they have been very ready to be enemies one to another. And those that embrace religion, must expect to meet with enemies in a world whose smiles seldom agree with Christ's. Recompense to no man evil for evil. That is a brutish recompence, befitting only animals, which are not conscious of any being above them, or of any existence hereafter. And not only do, but study and take care to do, that which is amiable and creditable, and recommends religion to all with whom you converse. Study the things that make for peace; if it be possible, without offending God and wounding conscience. Avenge not yourselves. This is a hard lesson to corrupt nature, therefore a remedy against it is added. Give place unto wrath. When a man's passion is up, and the stream is strong, let it pass off; lest it be made to rage the more against us. The line of our duty is clearly marked out, and if our enemies are not melted by persevering kindness, we are not to seek vengeance; they will be consumed by the fiery wrath of that God to whom vengeance belongeth. The last verse suggests what is not easily understood by the world; that in all strife and contention, those that revenge are conquered, and those that forgive are conquerors. Be not overcome of evil. Learn to defeat ill designs against you, either to change them, or to preserve your own peace. He that has this rule over his spirit, is better than the mighty. God's children may be asked whether it is not more sweet unto them than all earthly good, that God so enables them by his Spirit, thus to feel and act.
20. if thine enemy hunger, &c.—This is taken from Pr 25:21, 22, which without doubt supplied the basis of those lofty precepts on that subject which form the culminating point of the Sermon on the Mount.
in so doing thou shalt heap coals of fire on his head—As the heaping of "coals of fire" is in the Old Testament the figurative expression of divine vengeance (Ps 140:10; 11:6, &c.), the true sense of these words seems to be, "That will be the most effectual vengeance—a vengeance under which he will be fain to bend" (So Alford, Hodge, &c.). Ro 12:21 confirms this.