But he who is joined to the Lord is one spirit with Him. (1 Corinthians 6:17)
The word “joined” is not in the past tense but the ongoing present tense. In other words, “He who is cleaving continually to the Lord is one with Him.” There is a spiritual union with God that is parallel to the physical union between a man and a woman. That is what the Scripture says. It is so real, so intimate, that the believer can cleave to God so that he is one with Him.
Just as Jesus is one with the Father, so believers can be one with the Son. “He who is joined to the Lord is one spirit.” I want to emphasize again that it is not a single event in the past tense. It is a continuing present: “He who is cleaving continually to the Lord.” Jesus lived by continual union with the Father. If the union had ever been broken, which it never was until the cross, He would have forfeited life. And you and I, as believers, live only insofar as we live in continual union with the Son, Jesus Christ. Living in union with Him, we are one spirit.
The primary activity of the human spirit is union with God. It is the only part of man that can be united directly with God. The great privilege we have with our spirits is union and communion with God.
The believer has to live by his union with Christ, just as Christ lived by His union with the Father. We are dependent for life, moment by moment, upon our union with Christ. Don’t ever trust in yesterday’s experience for today.