Hebrews 13:3-6
2 “Remember them that are in bonds, as bound with them, them which suffer adversity as being yourselves also in the body. Marriage is honorable in all, and the bed undefiled; but whoremongers and adulterers God will judge. Let your conversation be without covetousness: being content with such things as you have.”
See how large is his discourse concerning chastity. “Follow peace,” he said, “and holiness; Lest there be any fornicator or profane person”; and again, “Fornicators and adulterers God will judge.” In every case, the prohibition is with a penalty. “Follow peace with all men,” he says, “and holiness, without which no man shall see the Lord: But fornicators and adulterers God will judge.”
And having first set down “Marriage is honorable in all men, and the bed undefiled,” he shows that he rightly added what follows. For if marriage has been conceded, justly is the fornicator punished, justly does the adulterer suffer vengeance.
Here he strips for the heretics. He did not say again, Let no one be a fornicator; but having said it once for all, he then went on as with a general exhortation, and not as directing himself against them.
“Let your conversation be without covetousness,” he says. He did not say, Possess nothing, but, “Let your conversation be without covetousness”: that is, let it show forth the philosophical character of your mind. [And it will show it, if we do not seek superfluities, if we keep only to what is necessary.] For he says above also, “And ye took joyfully the spoiling of your goods.” He gives these exhortations, that they might not be covetous.
“Being content” (he says) “with such things as you have.” Then here also the consolation; “For He” (he says) “has said, I will never leave you nor forsake you”; “so that we may boldly say, the Lord is my helper, and I will not fear what man shall do unto me.” Again consolation in their trials.
Source: Homilies on the Epistle to the Hebrews (New Advent)