He that is joined to the Lord. He has added this to show that our connection with Christ is closer than that of a husband and wife, and that the former, accordingly, must be greatly preferred before the latter, so that it must be maintained with the utmost chastity and fidelity. For if he who is joined to a woman in marriage ought not to have illicit connection with an harlot, much more heinous were this crime in believers, who are not merely one flesh with Christ, but also one spirit Thus there is a comparison between greater and less.

But he that is joined to the Lord - The true Christian, united by faith to the Lord Jesus; see John 15:1 ff.
Is one spirit - That is, in a sense similar to that in which a man and his wife are one body. It is not to be taken literally; but the sense is, that there is a close and intimate union; they are united in feeling, spirit, intention, disposition. The argument is beautiful. It is, "As the union of souls is more important than that of bodies; as that union is more lasting, dear, and enduring than any union of body with body can be, and as our union with him is with a Spirit pure and holy, it is improper that we should sever that tie, and break that sacred bond, by being joined to a harlot. The union with Christ is more intimate, entire, and pure than that can be between a man and woman; and that union should be regarded as sacred and inviolable." O, if all Christians felt and regarded this as they should, how would they shrink from the connections which they often form on earth! Compare Ephesians 4:4.

Is one spirit - He who is united to God, by faith in Christ Jesus, receives his Spirit, and becomes a partaker of the Divine nature. Who can change such a relationship for communion with a harlot; or for any kind of sensual gratification? He who can must be far and deeply fallen!

He that is joined unto the Lord,.... As every elect person is; his whole person, soul and body, is united to the Lord Jesus Christ, to his whole person, as God-man and Mediator; even as Adam and Eve, whose marriage was a representation of the marriage between Christ and his church, were personally united, and were called by the same name; and as the whole human nature of Christ, consisting of a true body and a reasonable soul, was united to the person of the Son of God; and as appears from the influence that union with Christ has upon the redemption, sanctification, and resurrection of the body. The ground, foundation, and bond of which union is, not the Spirit on Christ's part; for the Spirit being received as a spirit of regeneration, sanctification, &c. is a fruit of union to Christ, and an evidence of it; nor faith on our part, which as a grace is not ours, but the gift of God, and is a fruit of union; nor is it of an uniting nature, but is a grace of communion; and the foundation of all its acts, as seeing Christ, going to him, receiving of him, walking on in him; &c. is a previous union to Christ; but it is the everlasting and unchangeable love of Christ to them, shown in his choice of them, in his covenant with his Father on their behalf, in his engaging for them as a surety, in assuming their nature, and acting, both in time and eternity, as the representative of them, which is the bond and cement of their union, and from which there can be no separation. This union is first discovered in the effectual calling, and will be more manifest hereafter. Now he that is in this sense united to Christ,
is one spirit; for this union is a spiritual one; it is complete and perfect; near and indissoluble; by virtue and in consequence of it, God's chosen ones come to have and enjoy the same spirit in measure, which Christ their head and Mediator has without measure: hence they have the Spirit of God, as a spirit of illumination and conversion, of faith and holiness, of adoption, and as the earnest, pledge, and seal of their future glory. And since so it is, fornication, which makes them one flesh with an harlot, ought studiously to be abstained from.

one spirit--with Him. In the case of union with a harlot, the fornicator becomes one "body" with her (not one "spirit," for the spirit which is normally the organ of the Holy Spirit in man, is in the carnal so overlaid with what is sensual that it is ignored altogether). But the believer not only has his body sanctified by union with Christ's body, but also becomes "one spirit" with Him (John 15:1-7; John 17:21; 2-Peter 1:4; compare Ephesians 5:23-32; John 3:6).

But he that is joined to the Lord - By faith. Is one spirit with him - And shall he make himself one flesh with an harlot?