life-giving union with Christ

"For with the heart one believes and is justified..." (Romans 10:10a ESV)

The heart is the chief thing in believing; for into the heart Christ is received, and in it he dwells by faith. The vital union between Christ and the believer is manifested and made known in the heart, and there it is cemented and established. With joy can the believer say, "My Beloved is mine, and I am his!" What happiness for me -- we are but one person in the eye of the law, and our vested interests are but one. What a blessed state is this! Christ gives himself freely to the believer, who also gives himself up in faith to Christ. Christ as the believer's surety has taken his sins upon himself, and the believer takes Christ's righteousness; for Christ makes over all that he has to the believer, who by faith looks upon it and makes use of it as his own; according to that express warrant for his so doing in 1 Cor. 3: 22, 23 -- "All things are yours, because you belong to Christ."

This living union between Christ and the believer is extensively treated in Scripture. Christ speaks of it in his prayer for his people, that "those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you." (John 17:20-21) And, "Whoever feeds on my flesh and drinks my blood abides in me, and I in him." (John 6:56). This indwelling is by faith: "That Christ may dwell in your hearts by faith." (Eph 3:17) And It is the role of the Holy Spirit to manifest this union to their hearts:  "In that day you will know that I am in my Father, and you in me, and I in you." (Jn 14:20). 

This union is also represented by several striking images, such as that of husband and wife, who are in law but one person, the husband being responsible for the wife's debts, and the wife sharing in the husband's honors and possessions. It is illustrated by the union between a building and the foundation upon which it stands secure, and by the connection between a tree and its branches, which live and grow by the sap received from the tree. Also, it is seen between the head of the body and its members, which by union with the head live and grow, being nourished in every part. Under these beautiful images the Scripture sets forth the reality and the blessed fruits of this union. The Holy Spirit makes it known to the believer, by enabling him to rely upon God's word as infallible truth, and to receive Christ's person as the almighty Savior. He strengthens and enables the believer to make use of Christ's fulness, and to live by faith upon him in all his offices, for the partaking of all his promised graces and blessings.

That faith which comes from the working of God always produces the knowledge and the fruits of this blessed union, and enables the soul to give itself up to Christ, that it may be one with him, not in a figurative metaphorical way, but as really and truly as the building is one with the foundation, as much one in vested interest as husband and wife, as one in influence, as the root and the branches, the head and the members. So that this is not an empty notion about Christ, or some clear knowledge of him, or a mere approving of his way of salvation; but it is an actual receiving of him into the heart for righteousness to justify, and to dwell and reign there to sanctify, receiving him as a perfect Saviour, and living upon him in his fulness; waiting upon him to be taught daily; trusting wholly for acceptance in his blood and righteousness; resting, relying, leaning upon his promised strength to hold out unto the end; and hoping for eternal life as the free gift of God through Jesus Christ our Lord. The saving faith thus receives Christ, and thus lives upon Christ.

Now, reader, examine and test yourself, whether you have this faith. Do you believe with your heart unto righteousness? You cannot live upon Christ, unless you are first in Christ. The knowledge of your union with him must be clear and plain before you can have a free and open communion with him. There must be faith, before there can be the fruits of faith. The believer must have clear evidence of his vested interest in Christ before he can live comfortably and happy upon Christ. Therefore, he must look well to the foundation and see there be no doubts left about his being settled upon it. Christ being the sure foundation, how can he safely build his salvation upon it, unless he first be satisfied that he is established upon it? 

(William Romaine, The Life of Faith, abridged and modernized)