The new life, brought about by regeneration, is evidenced in the fact that God dwells in the regenerated man, while the regenerated man dwells in God. The regenerated man is born to the life of the spiritual world. He has returned to the original state of man when God in a special sense dwelt in man. With justification and regeneration the subjective restoration of man has begun to be realized, so that the image of God comes to be restored more and more. The Triune God is not far from His children, indeed He dwells in them. This indwelling in the hearts of the believers possesses greater significance than the general presence of God. The mystical union is a union with God that is more intimate and more peculiarly operative than the presence of God spoken of by Paul in Acts 17: 27, 28, where he says: “Though he is not far from each one of us: for in him we live, and move, and have our being.” Although it is not easy, indeed hardly possible, in anything like an adequate manner to describe the mystical union, still the doctrine is well founded in the Word of God and forms one of the most precious treasures of the Lutheran Church and a source of rich comfort to the believer.
1. The Definition of the Mystical Union.
Hollazius defines the mystical union as follows: “the Mystical Union is the spiritual conjunction of the Triune God with justified man, by which He dwells in him as in a consecrated temple by His special presence, and this substantial, and operates in the same by His gracious influence.” Unio mystica may be considered both as unitio and unio. Unitio is the act of union which is momentary and takes place at the same time as justification and regeneration. Unio is the continuous state. Dogmaticians also speak of the means of the mystical union. On the divine side these are the Gospel, Baptism and the Lord’s Supper. On the human side the means is faith. The union is called mystical because it is a great mystery, while its modus is not completely comprehensible.
Quenstedt presents both the mystical union with the Triune God and the special union with the God-man. He says that the latter implies that Christ constitutes the spiritual union with the regenerated man, works in and through him, so that what the believer experiences, suffers, and does as a Christian, is all dedicated to Christ. Compare Gal. 2: 20: “Christ liveth in me.” He says that through this union a Christian becomes anointed and furnished as a spiritual prophet, priest and king. This union is likewise a marriage covenant with Christ, so that the Christians become the bride of Christ. Compare Eph. 5: 31, 32.
Among other Scripture passages we would call attention to the following: “If a man love me, he will keep my word: and my Father will love him, and we will come unto him, and make our abode with him” (John 14: 23) ; “I come unto you” (John 14: 18; cf. 15: 4, 5; 17: 21—23; Rom. 8: 9—11) ; “Know ye not that ye are a temple of God, and that the Spirit of God dwelleth in you?” (1 Cor. 3: 16; 6: 19; cf. also 6: 15, 17; Gal. 3: 27); “A habitation of God in the Spirit” (Eph. 2: 22) ; “Partakers of the divine nature” (2 Peter 1: 4). The mystical union is presented in 1 John 4:16 as a mutual covenant of love: “We know and have believed the love which God hath in us. God is love; and he that abideth in love abideth in God, and God abideth in him.” The mystical union, therefore, not only finds expression in reciprocal love to God, but also love to all who are united with Christ. Cf. Eph. 5: 29, 30: “For no man ever hated his own flesh; but nourisheth and cherisheth it, even as Christ also the church; because we are members of his body.” Compare Eph. 4: 2—6; 1 Cor. 12: 26. “Now ye are the body of Christ, and severally members thereof” (1 Cor. 12: 27).
2. Negative and Positive Characteristics.
Inasmuch as the mystical union cannot be comprehended and described in an adequate way, therefore negative and positive terms are employed in order to obviate misunderstandings and as nearly as possible present the content of this doctrine. The negative characteristics are the following: 1) Non transsubstantialis (not transubstantiation). The believer is indeed a child of God, but is not made divine. The believers partake of the nature of God, but are not changed to a divine nature. 2) Non consubstantialis (not consubstatiation) so that two substances become one substance. 3) Non substantialis formaliter (not substantial form), i. e., like a grafted branch forms a unit with the tree. God dwells in the Christian, but the abode is not changed to the Indweller, nor vice versa. God can take His departure from man and therefore His indwelling is not an incarnation. Through the mystical union we put on Christ, but that which is put on is not identical with the person upon whom it is put. The followers of Weigel and Schwenkfeld taught that the union was essential. When Hollazius employs the term substantial he simply means that the divine substance is united with the human substance in a real although a mystical manner. 4) Non mere moralis (not a mere moral union), as for instance the union between the souls of David and Jonathan, since the union, implies a great deal more. 5) Non mere operatio gratiosa, since it is not only a divine activity. It is God Himself that dwells in man, not only His gifts. The positive terms are the following: 1) Vera et realis (true and real), since it is a true and real union and not one that is metaphoric and ideal. 2) Intima (intimate), so that God approaches the believer and enters into a special relationship with him. In a repletive sense God is omnipresent and can therefore enter into a special mystical union with the believers. He fills them with all the fullness of God, operates in and through them with all wisdom and power. This is a concursus in a higher degree. 3) Gratiosa in the Church militant or the kingdom of grace. 4) Gloriosa in the Church triumphant.3. Testimonium Spiritus Sancti Internum.
Since God in accordance with unio mystica dwells in man, and man becomes the temple of the Holy Spirit, therefore the Christian must in some way experience the mystical union and receive some testimony that he is a child of God. The life-relationship with the Lord is capable of revealing itself in a palpable way. There are many who at the beginning of the life of faith experience unusual joy and happiness, while the majority experience at least some moment of exalted glory when the assurance of faith is powerful and strong. But this experience is not the same in all men. It may happen in the case of some that quite some time will elapse before they come to know of the assurance of faith with its transport of heavenly joy. The emotions vary and the child of God soon learns that the Christian life ofttimes implies a struggle without the presence of any joyous emotions to comfort and cheer. On this account it is necessary that some guidance be afforded so that the Christian may know and be assured that he is a child of God. The method by which this is accomplished according to the Word of God is the testimony of the Holy Spirit or testimonium Spiritus sancti internum.
Paul writes in 1 Cor. 2: 10—12: “But unto us God revealed them through the Spirit: for the Spirit searcheth all things, yea, the deep things of God. For who among men knoweth the things of man, save the spirit of man, which is in him? even so the things of God none knoweth. save the Spirit of God. But we received, not he spirit of the world, but the spirit which is from God; that we might know the things that were freely given to us of God.” And in Rom. 8: 16 it is expressly stated: “The Spirit himself beareth witness with our spirit, that we are the children of God.”
Before the operation of the Spirit, as stated in Ordo Salutis, and especially before the new birth man knows God the Father only as a providential Father and usually only as the almighty and just judge of earth and heaven; Christ is known historically and the knowledge of the Holy Ghost is vague and His personality is not clear. By the co-witnessing of the Spirit the personality of the Holy Spirit becomes more and more distinct. The Holy Spirit reveals Christ through the Word in a real way and Jesus Christ becomes like a friend, yea, as a brother, and through Christ the Father becomes a real Father. A Christian understands then the meaning of the resurrection greeting of Jesus Christ: “Go unto my brethren and say to them, I ascend unto my Father and your Father, and my God and your God.” In the new birth or regeneration a believer enters the spiritual world of the kingdom of God in its first form of experience. The great Christian facts become clearer and clearer. The co-witnessing of the Spirit with our spirit becomes a strong testimony. The Bible as a textbook becomes a living guide and the Christian experience is clarified.
The testimony of the Spirit has been described in various ways. The most prominent theologians hold different views concerning the proofs of Christian experience and testimony. Among these theologians may be mentioned especially Frank, Philippi and Dorner. Frank bases the testimony of Christian experience on the great transformation through which the Christian passes in conversion and regeneration together with what he experiences in the daily conversion or sanctification. Dornert opposes the viewpoint of Frank, which he characterizes as subjective, i. e., according to Dorner, Frank has presented a subjective and not an objective principle of knowledge. Dorner declares that we may possess an immediate knowledge of God, not merely a secondary knowledge obtained through ratiocination which leads us back to the cause. He says that we do not become assured of God on the ground of our consciousness of regeneration and conversion, but because we know that God in Christ is for us, therefore we know that we are saved. Furthermore, faith possesses a spiritual intuition concerning God as our Father; it possesses knowledge not only concerning itself as redeemed, but also, and in a primary sense, concerning the God of our salvation. Dorner’s doctrine in regard to an immediate intuition does not correspond to true mysticism, it tends toward the false. Philippij sets forth the objective reconciliation performed by Christ, as attested an offered in the Word of God, both as the starting point and the only foundation on which a Christian can base his assurance of salvation. Nohrborg states that the testimonies of the Holy Spirit are twofold: internally in the heart and externally in the Word. In accordance with the first class of testimonies the Spirit bears witness through all His gracious acts taken together and He bears witness with our spirit, not alone. The effects which belong to the testimonies of the Holy Spirit begin with the gift of faith and continue on down through daily sanctification. In accordance, therefore, with the continued acts of the Spirit, we know that we are the children of God. Compare 1 John 3: 24: “And hereby we know that he abideth in us, by the Spirit which he gave us.” The Spirit bears witness with our spirit through the Word when we read and hear it, thereby learning to know the character of those that are the Lord’s. It is, therefore, not a new revelation, but the Spirit, dwelling in the believers and bearing testimony through the Word, that is the ground of faith and grants the internal assurance through His works of grace. This testimony of the Spirit will at times become especially clear, giving rise to great joy in the Lord. The Christian is also admonished in the Scriptures to seek the assurance of faith and joy in the Lord. But even if these experiences of joy be infrequent, still he relies and rests in faith on the promise of God in the Word.
 Hollazius: “Unio mystica est conjunctio spiritualis Dei triunius cum homine justificato. qua in hoc velut consecrato templo praesentia speciali eaque substantiali habitat et gratioso influxu in eodem operatur.”
 Compare Lindberg’s Apologetics. §19. 3, p. 151. ** System der Chr. Gewisshelt, S§ 15, 16. t System of Chr. Doctrine, Vol. I, pp. 31—184. t Glaubenslehre, V. 2. Zweite aufl., p. 58.
 Nohrborg, Postilla, 18th ed., p. 474.