romans 12:20 Therefore if thine enemy hunger, feed him; if he thirst, give him drink: for in so doing thou shalt heap coals of fire on his head. - Free Bible Online

      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.

JFB.