1 Corinthians 6:17
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 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.