1 Corinthians 6:19-20
3 “Do you not know that your body is a temple of the Holy Ghost which is in you?” He did not merely say, “of the Spirit,” but, “which is in you;” which was the part of one who also was soothing. And again, explaining himself still further, he added, “which you have from God.” He mentioned Him that gave also, both exalting the hearer and putting him in fear, both by the magnitude of the deposit, and by the munificence of Him that made it.
“And you are not your own.” This is not only to abash, but even to force men towards virtue. “For why,” says he; “doest thou what you will? You are not your own master.” But these things he said, not to take away free-will. For so in saying, “All things are lawful for me, but not all things are expedient,” he does not take away our liberty. And here again, writing, “You are not your own;” he makes no infringement upon freedom of choice, but he leads away from vice and indicates the guardian care of the Lord. And therefore he added, “For you were bought with a price.”
But if I am not my own, upon what ground do you demand of me duties to be done? And why do you go on to say again, “Glorify God therefore in your body and in your spirit, which are God's?” What then is the meaning of, “you are not your own?” And what does he wish to prove thereby? To settle them in a state of security against sin, and against following the improper desires of the mind. For indeed we have many improper wishes: but we must repress them, for we can. And if we could not, exhortation would be in vain. Mark, accordingly, how he secures his ground. For having said, “You are not your own,” he adds not, “But are under compulsion;” but, “You were bought with a price.” Why do you say this? Surely on another ground, one might say perhaps, you should have persuaded men, pointing out that we have a Master. But this is common to the Greeks also together with us: whereas the expression, “You were bought with a price.” belongs to us peculiarly. For he reminds us of the greatness of the benefit and of the mode of our salvation, signifying that when we were alienated, we were “bought:” and not simply “bought,” but, “with a price.”
“Glorify then, take up and bear, God in your body, and in your spirit.” Now these things he says, that we may not only flee fornication in the body, but also in the spirit of our mind abstain from every wicked thought, and from driving away grace.
“Which are God's.” For as he had said “your,” he added therefore, “which are God's:” continually reminding us that all things belong to the Lord, both body and soul and spirit: For some say, that the words “in the spirit” mean the gracious Gift; for if That be in us, God is glorified. And this will be, if we have a clean heart.
But He has spoken of these things as God's, not only because He brought them into being, but also because, when they were alienated, He won them again a second time, paying as the price, the blood of the Son. Mark how He brought the whole to completion in Christ, how He raised us up into heaven. “You are members of Christ,” says he, “you are a temple of the Spirit.” Become not then “members of a harlot:” for it is not your body which is insulted; since it is not your body at all, but Christ's. And these things he spoke, both to make manifest His loving-kindness in that our body is His, and to withdraw us from all evil license. For if the body be another's, “you have no authority,” says he, “to insult another's body; and especially when it is the Lord's; nor yet to pollute a temple of the Spirit.” For if any one who invades a private house and makes his way revelling into it, must answer for it most severely; think what dreadful things he shall endure who makes a temple of the King a robber's lurking place.
Considering these things therefore, reverence thou Him that dwells within. For the Paraclete is He. Thrill before Him that is enfolded and cleaves unto you; for Christ is He. Have you indeed made yourself members of Christ? Think thus, and continue chaste; whose members they were, and Whose they have become. Erewhile they were members of an harlot, and Christ has made them members of His own Body. You have therefore henceforth no authority over them. Serve Him that has set you free.
For supposing you had a daughter, and in extreme madness had let her out to a procurer for hire, and made her live a harlot's life, and then a king's son were to pass by, and free her from that slavery, and join her in marriage to himself; you could have no power thenceforth to bring her into the brothel. For you gave her up once for all, and sold her. Such as this is our case also. We let out our own flesh for hire unto the Devil, that grievous procurer: Christ saw and set it free, and withdrew it from that evil tyranny; it is not then ours any more but His who delivered it. If you be willing to use it as a King's bride, there is none to hinder; but if you bring it where it was before, you will suffer just what they ought who are guilty of such outrages. Wherefore you should rather adorn instead of disgracing it. For you have no authority over the flesh in the wicked lusts, but in those things alone which God may enjoin. Let the thought enter your mind at least from what great outrage God has delivered it. For in truth never did any harlot expose herself so shamefully as our nature before this. For robberies, murders, and every wicked thought entered in and lay with the soul, and for a small and vulgar hire, the present pleasure. For the soul, being mixed up with all wicked devices and deeds, reaped this reward and no other.
However, in the time before this, bad though it were to be such as these, it was not so bad: but after heaven, after the King's courts, after partaking of the tremendous Mysteries, again to be contaminated, what pardon shall this have? Or, do you not think that the covetous too, and all those whom he recounted before, have the Devil to lie with them? And do you not judge that the women who beautify themselves for pollution have intercourse with him? Why, who shall gainsay this word? But if any be contentious, let him uncover the soul of the women who behave in this unseemly manner, and he will surely see that the wicked demon closely entwined with them. For it is hard, brethren, it is hard, perchance even impossible, when the body is thus beautified, for the soul to be beautified at the same time: but one must needs be neglected, while the other is cared for. For nature does not allow these to take place together.
4. Wherefore he says, “He that is joined to a harlot is one body; but he that is joined to the Lord is one Spirit.” For such an one becomes thenceforth Spirit, although a body envelope him. For when nothing corporeal nor gross nor earthly is around him, the body does but merely envelope him; since the whole government of him is in the soul and the Spirit. In this way God is glorified. Wherefore both in the Prayer we are commanded to say, “Hallowed be Your Name:” and Christ says also, “Let your light shine before men, that they may see your good works, and glorify your Father which is in heaven.”
Source: Homilies on First Corinthians (New Advent)