For thou shalt heap coals of fire upon his head, and the LORD shall reward thee.
Doing good to personal enemies is right and wise (Pr 25:21). It proves you are a child of God, for you show His general kindness, even to those who hate and mistreat you. It is the best way to treat enemies, for it either melts their hatred or hardens their wickedness. But regardless of their response to your kindness, the LORD will bless you for the deeds.
God's law to Israel by Moses included kindness to personal enemies (Ex 23:4-5). But the Pharisees, profanely wicked in their depraved hearts, taught the opposite. They taught that men should love their neighbors, their friends, and hate their enemies (Matt 5:43). But the Lord Jesus Christ defied their humanistic thinking and taught perfect godliness.
He said, "But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you." (Matt 5:44). God shows daily natural kindness to His enemies, so such actions prove we are the children of God (Matt 5:45). Loving only your friends is proof of nothing, for all sinners do the same (Matt 5:46-47). Perfect men will love their enemies (Matt 5:48).
When natural men, with wicked hearts from Adam, have personal enemies, they think of revenge. But God condemns such thoughts (Rom 12:17-19; I Thess 5:15). He commands you to treat your enemies well (Lev 19:18; Rom 12:20-21). This has always been the character of godly men (Job 31:29-30; Ps 35:11-14). The Good Samaritan showed great kindness to a racial and religious enemy - a Jew (Luke 10:25-37; John 4:9; 8:48).
Two things happen from such noble action to enemies. First, you heap coals of fire on their heads. This metaphor refers to the ancient practice of smelting ore for precious metals. Applying heat to ore would cause the desired metals to melt and run free from the slag and dross. While ore was always put over fire for obtaining the precious contents, blazing coals were sometimes poured over the ore to increase the heat even more.
Genuine and sincere kindness, unnatural and unexpected, may melt a personal enemy (Pr 15:1). It may draw forth the precious metals of guilt, shame, and repentance. It may cause him to reconsider his hard treatment of you, feel the hot pains of remorse, and beg for forgiveness. David's kindness to Saul melted his hard heart on two occasions (I Sam 24:1-22; 26:1-25), and Gideon's praise of Ephraim calmed their anger (Judges 8:1-3).
Gracious kindness toward an enemy may also harden his heart in wickedness, if there is not any natural mercy there. The smelter's fire often reveals the total absence of precious metals. As the high heat hardens the refuse slag and burns up the dross, so the gentle benevolence of good men may harden the profanely wicked. The very gracious words of Jesus did not melt hearts in Nazareth (Luke 4:16-30), and His healing of an ear in Gethsemane did not slow the Jew's evil plans there (Luke 22:51).
Whether the coals of fire melt an oppressor's heart or harden it in mischief, the LORD will reward the man who treats his personal enemies with kindness. The blessed God sends his sunshine and rain on both His friends and enemies, and He is pleased when His saints do the same. Though an enemy's persecution is painful, the LORD will bless you.