"That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me" (John 17:21).
Though it be secret and mystical, yet it [our union with Christ] is real; because a thing is spiritual, it doth not cease to be real. These are not words, or poor empty notions only, that we are united to Christ; but they imply a real truth. Why should the Holy Ghost use so many terms; of being planted into Christ? Rom. 6:5, 'For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection;' of being joined to Christ? 1 Cor. 6:17, 'He that is joined to the Lord is one Spirit;' of being made partakers of Christ? Heb. 3:14, 'For we are made partakers of Christ, if we hold the beginning of our confidence steadfast to the end.' Do these terms only imply a relation between us and Christ? No; then the emphasis of the words is lost. What great mystery in all this. Why is this mystery so often spoken of? Christ is not only ours, but 'he is in us, and we in him.' God is ours, and we dwell in God: 1 John 4:13, 'Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit;' and ver. 15, 'Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God.' It is represented by similitudes, that imply a real union as well as a relative, by head and members, root and branches, as well as by marriage, where man and wife are made one flesh. It is compared here with the mystery of the Trinity, and the unity of the divine persons. It is not a notion of scripture, but a thing wrought by the Spirit.