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There is a mystical union of God and the believer, which is taught in the Scriptures and experienced by the Christian, but which is difficult to describe. Chronologically its beginning coincides with regeneration and justification; logically it follows upon them, and forms the next stage in the order of salvation. It is not to be interpreted simply as an activity of God in us, but possesses the nature of a personal fellowship (1 John 1:3). God lives in the believer, and the believer in God. It is the starting point and living source of that progressive sanctification which begins in the justified man and continues to the end of his earthly life.
(This doctrine is not contained in the Augsburg Confession or in the Apology; and the Formula of Concord barely touches it. It was developed by the later dogmaticians, [Abraham] Calovius, [Johann Andreas] Quenstedt, [Johann Friedrich] Koenig and [David] Hollazius, to guard against the pantheistic conceptions of the mystics, and at the same time to do justice to the partial truth contained in the false doctrines of [Kaspar] Schwenkfeld, [Valentin] Weigel and [Andreas] Osiander. The Formula of Concord does not develop the idea of the mystical union, but has this to say: “For although in the elect, who are justified by Christ and reconciled with God, God the Father, Son and Holy Ghost, who is eternal and essential righteousness, dwells by faith (for all Christians are temples of God the Father, Son and Holy Ghost, who also impels them to do right); yet this indwelling of God is not the righteousness of faith of which Paul treats and which he calls the righteousness of God; but it follows the preceding righteousness of faith, which is nothing else than the forgiveness of sins and the gracious acceptance of the poor sinner alone for the sake of Christ’s obedience and merit.” It rejects the teaching “that not God Himself but only the gifts of God dwell in the believer.” The mystical union is defined by Hollazius as “the spiritual union of the Triune God with the justified man, by which He dwells in him as in a consecrated temple with a special presence, and that a substantial one, and operates in him by a gracious influx.”)
The Scriptures teach not only that by faith man is justified and forgiven, but that Christ dwells in him, and through Christ the Holy Trinity. St. Paul declares of the Christians that they are in Christ (Rom. 8:1) and again that Christ is in them (Gal. 2:20). They live in fellowship or communion with God (1 John 1:3). Not only does the Holy Spirit dwell and work within them, so that they have the earnest of the Spirit in their hearts (1 Cor. 1:22), the witness of the Spirit that they are God’s children (Rom. 8:16) and the sealing with the Spirit of promise (Eph. 1:13), but the Father and the Son also come to the believers and take their abode in them (John 14:23). Christ is in the believers (Col. 1:27) and they in him (Rom. 8:1). As many as have been baptized into Him have put on Christ (Gal. 3:27) and are in the Lord (Rom. 16:11) and are made nigh because they are in Him (Eph. 2:13) and are free from condemnation (Rom. 8:1). They are members of His body, of His flesh and of His bones (Eph. 5:30), members of Christ (1 Cor. 6:15) and partakers of the divine nature (2 Pet. 1:4). Christ lives in them (Gal. 2:20) and dwells in their hearts by faith (Eph. 3:17), is in them (Rom. 8:10), and is to be formed in them (Gal. 4:19). The believers are members of His body (Rom. 12:4,5); they are united with Him as the branch with the vine (John 15:5), and their life is His life flowing through them.
A Mystery. This union, as its name indicates, is a mystery. It is experienced by the believer, but cannot adequately be put into words. The fullness of the experience is proportioned to the degree of faith and sanctification. The union is established when the sinner comes to faith and is justified, and grows more close, intimate and strength-giving as his sanctification increases. The spiritual life which he leads has its source and vitality in Christ. Believers live in Christ, and He in them, and His life flows into and through them. Without Him they can do nothing (John 15:5).
The source of all spiritual life is in God through Christ. By faith the believer is reunited with God from whom he was separated and cut off by sin. Thus he who was spiritually dead is now made spiritually alive. As the severed branch which is grafted back into the tree lives again because of its new union with the tree, so the believer lives again because of his union with God through Christ. The branch grows and puts forth leaves and fruit; but it does so only because and as long as it is vitally united with the tree from which its life comes. The believer lives and bears fruit in holy living; but he does so only because and as long as he is united with God by faith. Through this mystical union life comes to him from God. Only by virtue of this union does he live spiritually. What this union meant to Paul he tells us when he says, “Nevertheless I live, yet not I, but Christ liveth in me; and the life which I now live in the flesh, I live by faith of the Son of God who loved me and gave Himself for me” (Gal. 2:20).The indwelling of God in the believer must not be understood in the pantheistic sense, as though the person of the believer were absorbed by Christ. On the contrary it is a close personal union in which the believer rests in Christ and draws strength from Him. Nor must the union be understood in such a way as to make man divine. The personality of man is not changed in any way, but it is united in a mystical and indescribable yet real and comforting way with Christ, or with God in Christ, so that Christ lives in him and he in Christ. The mystery of this union finds its explanation in the faith which grasps Christ and makes Him its very own, and in the love which flows from that faith and binds the soul and Christ together in the most intimate and loving fellowship.
(Luther has many mystical elements in his writings. He says in his commentary on Galatians, 2:20: “Christ therefore, joined and united unto me and abiding in me, liveth this life in me which I now live. Yes, Christ Himself is this life which now I live. Wherefore Christ and I in this behalf are both one. … So Christ, living and abiding in me, taketh away and swalloweth up all evils which vex and afflict me. … Because Christ liveth in me, therefore look what grace, righteousness, life, peace and salvation is in me; it is His, and yet notwithstanding the same is mine also by that inseparable union and conjunction which is through faith; by the which I and Christ are made as it were one spirit. … Thou art so entirely and nearly joined unto Christ, that He and thou are made as it were one person; so that thou mayest boldly say, I am now one with Christ, that is to say, Christ’s righteousness, victory and life are mine. And again Christ may say, I am that sinner, that is, his sins and death are mine, because he is united and joined unto me and I unto him. For by faith we are so joined together that we are become one flesh and one bone [Eph. 5:31], we are members of the body of Christ, flesh of His flesh and bone of His bone; so that this faith doth couple Christ and me more near together than the husband is coupled with the wife.”)
Though the mystical union cannot be fully described because it is a mystery, it is nevertheless not to be regarded as a figure of speech, but as a reality. It is not to be understood as denoting merely that harmony has been established between man’s will and God’s, or that there simply exists a union of God and man in love, such as might exist between two human persons. Nor is it to be understood as denoting merely that the believer receives special and peculiar gifts from the Holy Spirit. It is a real indwelling of God in man, a real union between them, which the old dogmaticians described as a union of substance with substance, but which they took care to guard against the notion that the divine and human substances are confused or amalgamated.
St. Paul, in speaking to the Athenians, refers to the natural union between man and God as the source of life (Acts 17:28). But the mystical union is carefully to be distinguished from the natural one spoken of by the apostle. It is a spiritual union. It is, of course, also to be distinguished from the personal union of God and man in Christ, and from the pantheistic notion that man is swallowed up in God.
(The Reformed deny the doctrine of the mystical union. [Albrecht] Ritschl regarded the doctrine as worthless and unsound, and called it “apocryphal.”)