THEOSISJohn Chrysostom
Next he shows how this truly happens with the fornicator: "Or do you not know," he says, "that he who is joined to a harlot becomes one body with her?" (1 Cor 6:16).
How is this known? "For," he says, "the two shall become one flesh." "But he who is joined to the Lord is one spirit" (1 Cor 6:17).
Having been joined to each other, two can no longer be two, but both become one. See how again he conducts the rebuke by the very names, using the words harlot and Christ.
"Flee fornication." He did not say, abstain from adultery, but flee, that is, strive diligently to keep far from this vice.
"Every sin that a man commits is outside the body, but the fornicator sins against his own body" (1 Cor 6:18).
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This is less important than what was said above. Yet, when speaking about fornicators, he rebukes their sin from every side and shows the gravity of their guilt by both great and small things.
The first is said for the more reverent, and the latter for the weaker.
Paul's wisdom is revealed precisely in this: he admonishes by presenting not only what is great, but also what is small, shameful, and indecent.
But you will say, does not the murderer, the covetous man, and the robber defile his hand? Everyone knows this.
But since it was not possible to say that there is no one worse than the fornicator, he portrays the gravity of this sin in another way.
He explains that by fornication the whole body becomes unclean. It is defiled as though it had fallen into a filthy vessel filled with impurity.
For this reason we have such a custom: after acts of covetousness and robbery, no one takes care to wash, but returns straight home.
After adultery, however, people go to wash, as though they had become wholly unclean. Thus conscience is especially ashamed of this sin.
Both sins are grave, covetousness and adultery, and both cast into Gehenna. Yet, acting with complete foresight, Paul shows the gravity of adultery by every means he could.