Rule for Interpreting Commands and Prohibitions
24 If the sentence is one of command, either forbidding a crime or vice, or enjoining an act of prudence or benevolence, it is not figurative. If, however, it seems to enjoin a crime or vice, or to forbid an act of prudence or benevolence, it is figurative. “Unless you eat the flesh of the Son of man,” says Christ, “and drink His blood, you have no life in you.” This seems to enjoin a crime or a vice; it is therefore a figure, enjoining that we should have a share [communicandem] in the sufferings of our Lord, and that we should retain a sweet and profitable memory [in memoria] of the fact that His flesh was wounded and crucified for us.
Scripture says: “If your enemy hungers, feed him; if he thirsts, give him drink;” and this is beyond doubt a command to do a kindness. But in what follows, “for in so doing you shall heap coals of fire on his head,” one would think a deed of malevolence was enjoined. Do not doubt, then, that the expression is figurative; and, while it is possible to interpret it in two ways, one pointing to the doing of an injury, the other to a display of superiority, let charity on the contrary call you back to benevolence, and interpret the coals of fire as the burning groans of penitence by which a man's pride is cured who bewails that he has been the enemy of one who came to his assistance in distress.
In the same way, when our Lord says, “He who loves his life shall lose it,” we are not to think that He forbids the prudence with which it is a man's duty to care for his life, but that He says in a figurative sense, “Let him lose his life”— that is, let him destroy and lose that perverted and unnatural use which he now makes of his life, and through which his desires are fixed on temporal things so that he gives no heed to eternal. It is written: “Give to the godly man, and help not a sinner.” The latter clause of this sentence seems to forbid benevolence; for it says, “help not a sinner.” Understand, therefore, that “sinner” is put figuratively for sin, so that it is his sin you are not to help.
Source: Christian Doctrine (New Advent)