1 Corinthians 6:17
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)