This phrase joined unto the Lord, is thought to be taken out of Deuteronomy 10:20: To him shalt thou cleave. He that hath attained to that mystical union which is between Christ and every one that is a true believer, is not essentially, but spiritually and mystically, one spirit with Christ; his spirit is united to the Spirit of Christ, and he is one by him in faith and love, and by obedience, Christ and he have one will, and he is ruled and governed by Christ: therefore you must take heed what you do in making your bodies the members of harlots, which they cannot be, and the members of Christ also.
Related Commentaries of 1 Corinthians 6:17
1 Corinthians 6:17
17 But he that is joined unto the Lord is one spirit.