Not in the passion of lust, like the Gentiles that know not God:

“Have no fellowship with such a man, lest ye perish along with him, even should he be thy father, thy son, thy brother, or a member of thy family. For says [the Scripture], "Thine eye shall not spare him." You ought therefore to "hate those that hate God, and to waste away [with grief] on account of His enemies." I do not mean that you should beat them or persecute them, as do the Gentiles "that know not the Lord and God; " but that you should regard them as your enemies, and separate yourselves from them, while yet you admonish them, and exhort them to repentance, if it may be they will hear, if it may be they will submit themselves. For our God is a lover of mankind, and "will have all men to be saved, and to come to the knowledge of the truth." Wherefore "He makes His sun to rise upon the evil and on the good, and sendeth rain on the just and on the unjust; " of whose kindness the Lord, wishing us also to be imitators, says, "Be ye perfect, even as also your Father that is in heaven is perfect."”

“"That each one of you know how to possess himself of his own vessel in sanctification and honor, not in the passion of lust, even as the Gentiles which know not God." He says, "That each one of you know how to possess himself of his own vessel." It is, then, a matter to be learnt, and that diligently, not to be wanton. But we possess our vessel, when it is pure; when it is impure, sin possesses it. And reasonably. For it does not do the things which we wish, but what sin commands. "Not in the passion of lust," he says. Here he shows also the manner, according to which one ought to be temperate; that we should cut off the passions of lust. For luxury, and wealth, and idleness, and sloth, and ease, and all such things, lead us on to irregular lust. "Even as the Gentiles," he says, "which know not God." For such are they who do not expect that they shall suffer punishment.”

“Above, the Apostle commended the faithful for their constancy in tribulations and other goods; here he admonishes them, for the future, to act well. And first he sets forth a general admonition; second he specifies, at "that you abstain." Concerning the first he does two things: first he sets forth his intention, second he assigns the reason for the admonition, at "that you may abound," etc. He says therefore: I have heard of your past goods, but for the future we ask, etc. He induces them, first, on his own part. And so he says "we ask." Ps. 121:6: "Ask the things that are for peace," etc. Likewise on Christ's part, and so he says "we beseech," etc. He beseeches, however, because they were perfect. 1 Tim. 5:1: "Do not rebuke an elder, but beseech him as a father." But what does he ask? "That just as," etc. The Apostle had taught them how they ought to walk in the common way of justice, which is through the commandments. Hence he says: "you have received," etc. Ps. 118:32: "the way of your commandments," etc. Likewise how they might please God in the way of counsels. Wis. 4:10: "Being pleasing to God, he was made beloved." Or how you should walk, namely through right action. Jn. 12:35: "Walk while you have the light." How you should please, namely through right intention. "So also you may walk" — that is, that you keep to the first doctrine, not withdrawing from it. Gal. 1:8: "But though we, or an angel from heaven, should preach to you besides that which we have preached to you, let him be anathema." The reason for the admonition is taken, first, from the fruit of keeping the admonition; second, from the admonition itself, at "for you know what," etc. He says therefore: although you are good, yet through the exercise of the commandments and counsels you will abound and progress. 2 Cor. 9:8: "God is able to make all grace abound in you." For charity is so great that there always remains that in which one may progress. Likewise, the things which you received from the admonition are honorable and useful. Ps. 18:8: "The law of the Lord is unspotted," etc. Prov. 6:23: "The commandment is a lamp, and the law a light, and the way of life." Therefore he says "what precepts" — that is, of what sort. And this received through the Lord Jesus, from him. 1 Cor. 11:23: "For I have received of the Lord that which also I delivered unto you," etc. Heb. 2:3: "which, since it had received its beginning to be declared through the Lord, from those who heard," etc. And these are these things. "This is the will of God, your sanctification" — as if he were to say: all the precepts of God are for this, that you may be holy. For holiness denotes cleanness and firmness. And all the precepts of God lead to these: that one be clean from evil, and firm in good. Rom. 12:2: "that you may prove what is the will of God" — namely, as explained through the precepts. Then, when he says "that you abstain," he admonishes in particular. And first he corrects them concerning certain disorders among them; second he urges them to the observance of good things, in chapter 5, at "concerning the times." Now there were three disorders among them: namely, of carnal vices as regards some, likewise of curiosity, likewise of sorrow over the dead; and therefore he treats of these. The second, at "but concerning charity." The third, at "we would not have you." Concerning the first he does two things: first he admonishes them to abstain from an immoderate appetite for carnal things; second he gives the reason, at "because the avenger." Again, the first is divided into two, because first he forbids lust; second, avarice. And he always joins these together, because each is concerned with a bodily object, although this is completed in spiritual delight. Likewise, first he teaches to beware of lust as regards one not one's own; second as regards one's own wife, at "that he may know." He says therefore "that you abstain from fornication." For it is the will of God to abstain from fornication; therefore it is a mortal sin, because it is against the precept and will of God. Tob. 4:13: "Keep yourself from all fornication." But also with respect to one's wife you should abstain honorably, so that each one may know how to possess his vessel — that is, his wife — "in sanctification," ceasing for a time, "and in honor, not in passion" — namely, so that passion should go before — "even as the Gentiles" — because this is the way of the Gentiles, to seek present delights, but not those of the future life. "In sanctification and honor," because here is the due use of marriage, which is for the good of offspring, or for rendering the debt, and thus it can be without sin; but sometimes it is a venial sin, if concupiscence is not carried beyond the bounds of marriage — namely when, although he has concupiscence, he would nevertheless not use it unless she were his wife. But when it is outside the bounds of marriage, mortal sin follows, and this is when, if she were not his wife, he would still use her, and more willingly with another. Heb. last [chapter], 4: "Let marriage be honorable, and the bed undefiled." "But fornicators and adulterers God will judge." 1 Pet. 3:7: "Husbands likewise dwelling with them according to knowledge, giving honor to the woman as to the weaker vessel, as to co-heirs of grace and life, that your prayers be not hindered." Then, when he says "and let no man," he forbids avarice. Hence he says "and let no man overreach" — that is, use violence by taking away another's goods through power. Jas. 2:6: "Do not the rich oppress you by might?" Nor circumvent by deceit. Jer. 5:27: "As a snare full of birds, so their houses are full of deceit." Then, when he says "because the avenger," etc., the reason for the admonition is given, which he first assigns from divine vengeance; second he shows this vengeance to be just, at "for he has not called." He says therefore: let us abstain from these things, because "the Lord is the avenger." Gal. 5:21: "I foretell you, as I have foretold you, that they who do such things shall not obtain the kingdom of God." For certainly he avenges justly, of which there is one reason from God calling, a second from the contrariety to the gift. If the Lord calls you to one thing, and you do the contrary, you are worthy of punishment. And therefore he says "for he has not called," etc. Eph. 1:4: "He chose us in him before the foundation of the world, that we should be holy and unspotted in his sight in charity." Rom. 8:30: "whom he called, them he also justified," etc. Therefore he says "therefore," etc.; as if to say: this is one special reason which I have stated. There is another reason, because these vices are contrary to the Spirit, who has been given to us. Hence, whoever does these things does injury to the Holy Spirit. Therefore he says "who also," etc. Heb. 10:28: "A man making void the law of Moses dies without any mercy, on the word of two or three witnesses; how much more, do you think, does he deserve worse punishments, who has trodden underfoot the Son of God, and has held the blood of the testament unclean in which he was sanctified, and has done despite to the Spirit of grace?" Then, when he says "but concerning charity," etc., he draws them back from idleness. Now it must be known that, as Jerome says in his letter to the Galatians, the Thessalonians were generous, and it was their custom for the rich to give much; and therefore the poor idly clung to their benefaction, not caring for labors, but running about through houses. And therefore first he commends the generosity of the givers; second he dissuades the idleness of the receivers, at "and that you give attention." He says therefore, first, that they do not need to be admonished to charity; second he admonishes that they progress in it, at "but we ask." He says therefore "but concerning the charity of brotherhood" — that is, that you love the brethren — "it is not necessary to write to you." Rom. 12:10: "Loving one another with the charity of brotherhood." Heb. last [chapter]: "Let the charity of brotherhood remain in you." And the reason is because "you yourselves have learned from God" — namely, the precept in the law. Lev. 19:18: "You shall love your friend as yourself." Likewise in the Gospel. Jn. 13:34: "A new commandment I give you, that you love one another," etc. Or, you have learned by interior instruction. Jn. 6:45: "Everyone who has heard from the Father, and has learned, comes to me," etc. And this he learns through the Holy Spirit. Then, when he says "but we ask," he exhorts them to progress in charity, saying: and because you have charity toward all, therefore we ask that you progress. And although others abuse it, you nevertheless should persist. Prov. 15:5: "In abundant justice virtue is greatest." Then, when he says "and that you give attention," he reproves the idle: first their restlessness; second he shows how they should repress it; third, why. He says therefore: "that you give attention, that you be quiet." Prov. 7:10: "talkative and wandering, impatient of quiet, nor able to remain in the house with her feet," etc. 2 Thess. 3:7: "We were not restless among you, neither did we eat any man's bread for nothing, but in labor and weariness, working night and day," etc. Repressing idleness by exercising business. Hence he says "and that you do your own business." Prov. 24:27: "Diligently work your field, that afterward you may build your house." He says, however, "your own" — but is it not the case that another's business is also to be done? And it seems that it is so. Rom. last [chapter]: "that you assist her in whatever business." I respond. It must be said that all things can be done in a disordered way, if they are done outside the order of reason — namely when someone conducts himself dishonestly — and in an ordered way, namely when the order of reason is kept, and in necessity, and this is commendable. "And that you work with your own hands" — that is, by working with your hands. Ecclus. 33:29: "Idleness has taught much evil," etc. Ezek. 16:49: "This was the iniquity of Sodom your sister: pride, fullness of bread, and abundance, and the idleness of her and of her daughters, and they did not reach out a hand to the needy and the poor," etc. And this is a precept for all those who do not have other means by which they can live lawfully; because it is a precept of nature that a man should sustain his body. 2 Thess. 3:10: "He who is not willing to work, let him not eat either." Of this there is a twofold reason. The first, on account of the example to others. Hence he says "that also honestly," etc. For unbelievers, seeing your manner of life so idle, detest you. 1 Tim. 3:7: "But he must have a good testimony from those who are outside," etc. The second reason, that you should not desire the things that are others'; hence it is said "and that you desire nothing of any man's." Prov. 21:25: "Desires kill the slothful." Eph. 4:28: "He who stole, let him now steal no more," etc. And therefore, if this restlessness is repressed, it results in good example, and in the repression of desire.”

“Not in the lust of concupiscence,.... Or "passion of lust"; for the mere gratifying and indulging of that; for a man so to possess his vessel, is to cherish the sin of concupiscence, the first motions of sin in the heart, by which a man is drawn away, and enticed; to blow up the flame of lust, and to make provision for the flesh to fulfil the lusts thereof: even as the Gentiles which know not God; for, though they knew him, or might know him with a natural knowledge, by the light and works of nature, yet they knew him not savingly and spiritually, as he is revealed in the word, of which they were destitute; or as the God of all grace, and the God and Father of Christ, or as he is in Christ: and though by the light of nature they might know there was a God, yet they knew not who that God was; nor did they act up to that light and knowledge they had; they did not glorify him as God, by ascribing to him what was his due; nor were they thankful for the mercies they received from him; nor did they fear, love, worship, and serve him; nor did they like to retain him in their knowledge, and therefore were given up to judicial blindness and hardness, to a reprobate mind, and to vile affections, and so did things very inconvenient, unnatural, and dishonourable. Wherefore, for a man to use either his wife or his body in any unchaste and dishonourable manner, for the gratifying of his lusts, is to act an Heathenish part; a like argument, dissuading from things unlawful, is used in Mat 6:32.”

“Not in the lust of concupiscence - Having no rational object, aim, nor end. Some say, "not like beasts;" but this does not apply as they who use it wish, for the males and females of the brute creation are regular and consistent in their intercourse, and scarcely ever exceed such bounds as reason itself would prescribe to those most capable of observing and obeying its dictates. The Gentiles which know not God - These are the beasts; their own brutes are rational creatures when compared with them. Enough has been said on this subject on Romans 1, and 2: They who wish to see more may consult Juvenal, and particularly his 6th and 9th Satires; and indeed all the writers on Greek and Roman morals.”