Whilst I have read various interpretations of Romans 12 v19-20 I'm not really convinced by any of them. Is the phrase 'heap burning coals on his head' idiomatic and if so what does it mean?
The two verses are set within a passage in which Paul is encouraging positive Christian virtues and skills and then comes an injunction which at face value seems to encourage acts of kindness which will ultimately harm rather than help.
When this passage from Romans is one of the RCL readings one can only imagine what members of our congregations make of it. Indeed, should it ever be read in public without some spoken or written comment on its meaning?
As others have noted, Paul is quoting here from Proverbs 25:21-22 and the phrase is an idiom. The issue comes up because we're not certain what that idiom means exactly, other than that both Paul and the writer of Proverbs believed that if you were kind to an enemy, they would respond in a way that both writers found was helpful in de-escalating a conflict with an enemy.
But what ancient practice does that draw from? What is that response, exactly? We don't really know the answer to the first question, although someone studying ancient cultures may one day tell us. There is also no consensus on what the idiom actually means, although it's helpful to remember that it is not talking about literally harming someone by placing burning coals directly on their heads. On the head is where many cultures, even today, carry things in containers suitable for the substance.
Commentators from the early church Fathers, to the Protestant reformers, right through today's scholars differ in their interpretations of the idiom and those interpretations break down into a number of categories:
1. It's a good thing. (Hey, thanks for the coals to keep the fire in my hearth going.)
2. It's a mean thing. (Well, now I feel like scum for being an enemy when you've been kind to me. And now I hate you more for making me feel bad.)
3. It's a clever means of protecting yourself. (Someone carrying burning coals on their heads can't haul off and punch you without endangering themselves.)
4. It's a cleansing fire. (My wickedness is burned away by your kindness and I will no longer be an enemy to you.)
5. It's a fire of love. (I deserve the treatment of an enemy, but you have given me the treatment of a friend. Your loving action has changed my mind about you. We can be reconciled.)
The work cited most frequently in discussing this is an essay by William Klassen called "Coals of Fire: Signs of Repentance of Revenge" in the ninth volume of New Testament Studies, pp. 337-350. I was not able to find the whole work online, but I did find someone who recorded his conclusion: "[T]he interpretation so widely accepted by interpreters that the coals of fire refer to shame, remorse, or punishment lacks all support in the text. In the Egyptian literature and in Proverbs the 'coals of fire' is a dynamic symbol of change of mind which takes place as a result of a deed of love." (p. 349)
John Calvin's words about this idiom in his commentary on Romans sums it up in practical terms: "Either our enemy will be softened by kindness, or, if he is so ferocious that nothing may assuage him, he will be stung and tormented by the testimony of his conscience, which will feel itself overwhelmed by our kindness."
With all those differing interpretations, we are left to think about the text in its context, both in Proverbs and in Romans as well as the other general teachings of Jewish and Christian faith. From "love your neighbor as yourself" in Leviticus 19:18 to "Love your enemies and pray for those who persecute you" in Matthew 5:44 and many other passages, it's clear throughout the Bible that whether enemies who receive our kindness have a change of heart or not, it's our own behavior, not theirs that should be our focus. We are to do the good and loving thing, whether it changes anything about the relationship or not. And it's clear from one glance around the world that, even apart from religious teaching, responding to hate with more hate makes things worse, not better.
My old Harper Study Bible (RSV) puts Rom. 12:20 in quotation marks and tells me that Paul is here quoting from Proverbs 25:21-22. Proverbs adds "and the LORD will reward you"; Paul adds the implied general practical maxim, "Do not be overcome by evil, but overcome evil with good," of which Rom. 12:20a ("if your enemy is hungry, feed him; if he is thirsty, give him drink") is just one example.
The business about "heap[ing] burning coals upon his head," in both Proverbs and Romans, is a metaphor, like having egg on one's face. It means that when, instead of returning evil for evil (revenge), you return good for evil, you will shame your enemy by not wreaking vengeance on him. Leave vengeance to God (Rom. 12:19); you show the divine mercy by returning good for evil, "and the LORD will reward you."
The interpretation that I like best and which fits the passage well is by Paul Meyer in the Harper Bible Commentary (1988). He notes an Egyptian ritual in which a basin of burning charcoal was carried on one’s head as a token of penitence. He refers to 4 Ezra 16.53 where the phrase is used in the context of admitting sins in order to cease from them (vv. 51, 67). They thus escape God’s judgment (vv. 64-67a). This fits the context of Rom.12.9-21 in which we are to reach out to others in love and humility, overcoming evil with good. Our caring for the basic needs of our enemies convicts them of their sin. Judgment is a matter for God, not us (v. 19), but by our love they repent of their sin and do not have to face God’s vengeance on their evil.
The general opinion of commentators seems to be that 'coals of fire on the head' is an idiom meaning 'repentance' - so by doing your enemy good you lead him/her to repent. I agree that, read in church without comment, the impression given is a vindictive one!
In Romans 12:20-21, Paul lifts wording rather directly from Proverbs 25:21-22 (the Septuagint form), including the “coals of fire on their heads” bit. Scholars have debated how the Greek translators of Proverbs came up with this phrasing, and various suggestions have been offered, with none (to my knowledge) winning a consensus.
In any case, Paul’s quoting of the phrase clearly focuses on the note of hospitality, even to enemies, and the promise of God’s reward for doing so. The bit about “coals of fire” is simply a part of the text quoted, and it’s unlikely that Paul meant any particular emphasis or meaning on that. The most that one might presume is that it was perhaps a figure of making an enemy feel some kind of shame over being treated so kindly.
In Romans 12:20 Paul is quoting Prov 25:22 in the context of arguing love (“overcome evil with good”) and leaving vengeance to God. There are four possibilities for meaning: “heap burning coals on his head” 1) used in 2 Esdr 16:53 to suggest God’s punishment of the sinner; 2) it is similar to a known Egyptian rite of contrition; 3) it can be retranslated “then you will be snatching coals (from) upon his head”; and 4) the fiery coals represent red-faced shame of the enemy in response to your kindness. Personally, I favor #1.