"The principle of nonretaliation for personal injury permeates the entire New Testament."369
12:16 The first part of this verse means "Have equal regard for one another"(NEB). Feelings of superiority are neither realistic nor appropriate for those who owe all to God's grace.
12:18 Paul strongly advocated being a peacemaker (cf. Matt. 5:9), but he did not promote peace at any price. In some situations, peace might give way to conflict if, for example, the truth is at stake. Notwithstanding the believer should not be the instigator of trouble under normal circumstances. Note Paul's two qualifiers regarding living at peace in this verse.
12:19 If hostility does erupt, the Christian should not retaliate. Rather he or she should trust God to right the wrong (cf. 1 Sam. 24-26). Long ago God promised to take care of His people when others wronged them (Deut. 32:35).
12:20 Instead of doing one's enemy an unkindness the believer should do him or her positive good (cf. Matt. 5:44). This may result in the antagonist acknowledging his error and even turning to God in repentance. Heaping burning coals on his head figuratively describes doing good that results in the conviction and shame of the enemy. The expression alludes to the old custom of carrying burning coals in a pan. When one's fire went out at home, a person would have to go to a neighbor and request hot coals that he or she would then carry home on the head. Carrying the coals involved some danger, discomfort, and uneasiness for the person carrying them. Nevertheless they were the evidence of the neighbor's love. Likewise the person who receives good for evil feels uncomfortable because of his neighbor's love. This guilt may convict the wrongdoer of his or her ways in a gentle manner.
12:21 Paul again concluded with a summary. Being overcome by evil means giving in to the temptation to pay back evil for evil. When people do wrong, they expect to receive evil from those they have wronged. When they receive kindness instead, their hard hearts often become softer.
There is a progression in 12:9-21. Paul progressed from the Christian's duty to his fellow believers to action that would affect non-Christians as well. However all that Paul wrote in 12:3-21 is directly applicable to life within the body of Christ. The believer may encounter enemies there as well as in the world.
The general nature of the commands in this pericope illustrates the essentially gracious character of the Law of Christ (Gal. 6:2) under which Christians now live. Compare this with the legal nature of the commands in the Mosaic Law (cf. 10:4). God gave the Israelites many explicit commands about how they were to behave in a multitude of specific situations. The commands in verses 9-21, as well as in all the New Testament, are much more general and are similar to principles. This is one reason we say the Israelites lived under "law"and we live under "grace."