This usage points out how easily a person can be misled or confused by an inference in contrast to a direct, concrete statement. From this verse, one could conclude that, if he is joined to the Lord, then he is a spirit just as the Lord is. "He who is joined to the Lord is one spirit with Him." The hat-pin test disproves this very quickly. We are not a spirit, not the way the Lord is a spirit.
When we read it in its wider context, Paul reveals that he is not writing on the theme of spirit composition at all. His theme is "closeness of connection," which he illustrates by a man being "joined to a harlot." Unity emerges as the theme as he brings Christ into the picture, and in this case, a Christian's unity with Him is the highest, purest form of unity that a human being can be involved in.
Paul is suggesting, then, that a sheep may wander from the shepherd, a branch may be cut from a tree, a limb severed from the body, a child alienated from his parents, and even a wife from her husband; but when two spirits blend into one, nothing can separate them. So close is their unity that what affects one affects the other. This is why Jesus says in Matthew 25:40, "Inasmuch as you did it to one of the least of these My brethren, you did it to Me."
So, Paul concludes, do not involve Christ in sin. We should do everything in our power to affect that intimate spiritual relationship, that unity, for good. Our unity with Jesus Christ is spiritual and so close that, as God looks at it, it is closer than being joined in intercourse with a harlot! The reason for this is that, even in such a situation as that, the man and woman are, in reality, still two beings.
However, if we are in Christ, we are actually in His body, which is why Paul employs the word "spirit." We cannot see His body. It is invisible, but it is real! We are in Him! Are we truly aware of that? We need to be growing in the understanding of it. We are cells in His body, as it were, and as Paul explains in I Corinthians 12:26, when one part of the body hurts, the whole body hurts. When one part of the body is strengthened, the whole body is strengthened.
We must begin to understand that, when God uses the word "spirit" in this way, it suggests a unity that is extremely close. It is a matter of the joining of minds!
John W. Ritenbaugh
The Holy Spirit and the Trinity (Part 1)
Adultery creates a second one-body/one-flesh bond in opposition to the marriage. This will inflict severe damage upon the marriage relationship. The apostle says such sexual sins hurt so much because they are "sins against [our] own body" (verse 18).
Paul comes to his primary point in verses 19-20: We are not our own! God bought us at an incredibly high cost, the blood of our Master, and thus He commands us to "glorify God in your body and in your spirit," both of which are His! God owns us completely!
The import of this is staggering! When we commit sex sins—even in our minds—we have first become unfaithful to God! When we break the seventh commandment, we show infidelity to God! Yes, it shows infidelity to the wronged spouse, but it all begins with unfaithfulness to God.
The road to adultery starts when we become willing to break the vows we made to God at our baptism. We promised then that we would honor and obey Him exclusively and faithfully, accepting Him as our Savior, Master, and soon-coming King and Husband. When we are willing to walk away from the commands He gives us about sex and marriage, we begin to walk into the arms of adultery. Physical adultery starts with spiritual adultery!
If an adulterer desires to repent, he must first acknowledge that he has sinned against God. King David, in his moving prayer of repentance after the murder of Uriah and adultery with Bathsheba, cries out, "Against You, You only, have I sinned, and done this evil in Your sight" (Psalm 51:4). Did he not also sin against Uriah, Bathsheba, the nation, his wives, and his children? Of course! But ultimately, his sin was against God! When we are faithful to God and our covenant with Him, we will not commit sex sins.
Sex, Sin and Marriage
Being in or in union with God does not mean to be bodily inside of each other, because in all the verses that describe people who were "in" another, they had bodies of their own. So being in means "joined with" toward the accomplishment of the same purpose, and in our case, it is for the fulfilment of God's purpose that we are in union with Him.
John W. Ritenbaugh
Image and Likeness of God (Part 4)
Other Forerunner Commentary entries containing 1 Corinthians 6:17:
1 Corinthians 3:16-17