Literal coals and literal head?
A Protestant pastor of mine once taught that in Bible days people would carry coals (burning coals, "coals of fire") on their heads in some sort of fireproof container. By heaping these on an enemy's head you are giving them a gift, a blessing, so they can start their own fire.
Needless to say, this sounded fishy to me.
How does feeding them equate with giving them some of your coals?
Would your enemy calmly stand there while you transferred coals from your head to his? (Wouldn't you both take the containers off your heads first?)
How is it possible to feed your enemy when in battle? (instead of pouring hot coals on them which is what you were going to do until you remembered Paul's words in this verse).
In the context, Paul is teaching about revenge. He gives the example of how to relate to your enemy as an illustration of his teaching that we should not vent our anger on others nor seek revenge. Instead, we should bless them and let God judge them. God will heap the coals of fire of his judgment on them as required; it is not our place to do this.
We should not harbor desires for revenge when we are wronged. God will judge everyone properly, hopefully forgiving them if they cry out to him for forgiveness.
Instead we should love our enemy as our neighbor, feeding them.
The second part is the hard part, bringing up images of fighting an enemy in battle. If you were inside a city under siege, you might pour down hot coals on the heads of those below trying to breach the walls.
Should we refuse to defend ourselves and our families, allowing our enemies to vanquish us? This verse is not teaching pacifism.
We don't conquer the powers of darkness by being wicked ourselves.
Other Bible passages using the phrase "coals of fire"...
Many interpret the phrase "heap coals of fire on the head" as figurative giving it all sorts of meanings such as...
Once we start interpreting like this, the Bible means nothing at all because it means anything you want. Sadly, this is mostly the kind of teaching and preaching I hear from fundamentalist evangelical Protestants on the radio and in churches.
In the very early days of the church too many early church fathers adopted figurative interpretation — even the apostles did it. The results are (then and now) you can support any wacky idea you want to with words and phrases figuratively meaning whatever you want.
So here are the literal underpinnings...
Ezekiel 10:2 illustrates what is going on. In judging Jerusalem, God commands an angel to scatter coals of fire over Jerusalem. In the spiritual, heavenly realm, literal coals are literally poured-out on the city. Back in the physical realm, the inhabitants of Jerusalem experience this in the form of various invasions by enemies.