I. The Nature and Essence of Union with Christ (The Mystical and Vital Bond)
"To be a member of Christ is to be engrafted into him, and to be united to him by the same Holy Spirit dwelling in him and in us, and by this Spirit to be made a possessor of such righteousness and life as is in Christ: and to be made acceptable to God on account of the righteousness of Christ imputed unto us by faith, in as much as this righteousness is imperfect in this life." [Zacharias Ursinus - Commentary on the Heidelberg]
"The relation which holds between the head and the members of the same body, is a most fit and striking illustration of the close and indissoluble union between Christ and us." [Zacharias Ursinus - Commentary on the Heidelberg]
"Christ is always present with us with respect to his majesty; ... but as it regards the presence of his humanity, ... Christ is, therefore, present with us, 1. By his Spirit and Godhead. 2. By our faith, and the confidence with which we behold him. 3. By mutual love; because we love him, and he loves us in such a way as not to forget us. 4. By union with his human nature; for it is the same Spirit which is in us and him, that unites us to him." [Zacharias Ursinus - Commentary on the Heidelberg]
"By faith we have union with Christ—and so the kindred comes in; being united to Christ, the natural Son, we become adopted sons. God is the Father of Christ; faith makes us Christ's brethren—and so God comes to be our Father." [Thomas Watson - The Lord's Prayer]
"That communion with God is a unio mystica; it far surpasses our understanding; it is a most intimate union with God through the Holy Spirit; it is a unio personarum, an indissoluble and eternal covenant of God and man, which is far too weakly described by the designation ethical and is therefore indicated as mystical..." [Herman Bavinck - Reformed Dogmatics]
"For he meant not to refer to the sense in which Adam used the words, but sets forth, under the figure and similitude of marriage, the sacred union which makes us one with Christ." [John Calvin - Institutes]
II. The Legal and Federal Basis of the Union
"The substance of this comfort consists in this that we are ingrafted into Christ by faith, that through him we are reconciled to, and beloved of God, that thus he may care for and save us eternally." [Zacharias Ursinus - Commentary on the Heidelberg]
"Therefore, the Word, who is the natural Son of God, is alone our Mediator, in whom, as in the first begotten, we are adopted as the sons of God, as it is said: 'If the Son shall make you free, ye shall be free indeed.'" [Zacharias Ursinus - Commentary on the Heidelberg]
"Already in eternity, in election, and more closely still in the pactum salutis, a bond has been laid between the mediator and those who have been given to him by the Father. Already then, in the decree of God, a unio mystica has been laid between both and a substitution has come about." [Herman Bavinck - Reformed Dogmatics]
"He has chosen us in him before the foundation of the world, that we should be holy and without blame before him in love: having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will, to the praise of the glory of his grace..." [John Calvin - Institutes]
"In the covenant of peace Christ voluntarily took upon Himself to be the Head and Surety of the elect, destined to constitute the new humanity, and as such to establish their righteousness before God by paying the penalty for their sin and by rendering perfect obedience to the law and thus securing their title to everlasting life." [Louis Berkhof - Systematic Theology]
III. The Holy Spirit as the Bond and Mediator of the Union
"He unites us to Christ, that we may be his members and be quickened by him, and so be made partakers of all his benefits." [Zacharias Ursinus - Commentary on the Heidelberg]
"The union of all the saints with Christ, as members with the head, which is effected by the Holy Ghost, who dwells in the head, and in the members, conforming and making them like unto their glorious Head..." [Zacharias Ursinus - Commentary on the Heidelberg]
"So long as Christ is separated from us we have no benefit from him. We must be ingrafted in him like branches in the vine. Hence the Creed, after treating of Christ, proceeds in its third article, I believe in the Holy Spirit,—the Holy Spirit being the bond of union between us and Christ." [John Calvin - Institutes]
"It is enough for us to know that Christ and his people are really one. They are as truly one as the head and members of the same body, and for the same reason; they are pervaded and animated by the same Spirit." [Charles Hodge - Systematic Theology]
"The union between Christ and us is that whereby Christ and we are joined together, and made one. ... We are united unto Christ-- 1. By the Spirit on God's part, whereby he draws us, and joins us unto Christ." [Thomas Vincent - The Shorter Catechism]
IV. The Instrumental Role of Faith in Subjective Union
"Those are true christians who are not only baptized and profess the doctrine of Christ, but who are also possessed of a true faith, and declare this by the fruits of repentance; or, they are those who are members of Christ by a true faith, and are made partakers of his anointing." [Zacharias Ursinus - Commentary on the Heidelberg]
"...for it is through faith that we receive the Spirit, through whom this union is effected. But the members are united with each other and among themselves by mutual love, which cannot be wanting if we are joined to the head..." [Zacharias Ursinus - Commentary on the Heidelberg]
"...faith is the grace of union, by which we may spell out our interest in God. Faith does not, as the mariner, cast its anchor downwards—but it casts its anchor upwards. ... Other graces make us like God; faith makes us one with him." [Thomas Watson - The Ten Commandments]
"We are united unto Christ-- ... 2. By faith on our part, whereby we come unto Christ, and lay hold upon him." [Thomas Vincent - The Shorter Catechism]
"Faith is first of all a gift of God, and as such a part of the treasures that are hidden in Christ. It enables us to appropriate on our part what is given unto us in Christ, and to enter ever-increasingly into conscious enjoyment of the blessed union with Christ, which is the source of all our spiritual riches." [Louis Berkhof - Systematic Theology]
"Therefore, flesh alone does not constitute the union of brotherhood. But although the apostle assigns to believers only the honour of being one with Christ, it does not however follow, that unbelievers have not the same origin according to the flesh; just as when we say that Christ became man, that he might make us sons of God, the expression does not extend to all classes of persons; the intervention of faith being necessary to our being spiritually ingrafted into the body of Christ." [John Calvin - Institutes]
V. The Sacramental Signification and Sealing of the Union (The Lord's Supper)
"It is not only to embrace with a believing heart, all the sufferings and death of Christ, and thereby to obtain the pardon of sin and life eternal; but also, besides that, to become more and more united to his sacred body, by the Holy Ghost, who dwells both in Christ and in us; so that we, although Christ is in heaven, and we on earth, are, notwithstanding, 'flesh of his flesh and bone of his bone;' and that we live and are governed for ever by one Spirit, as members of the same body are by one soul." [Zacharias Ursinus - Commentary on the Heidelberg]
"The sacramental union is nothing else, than the relation which the sign has to that which is signified, from which we obtain this infallible rule: While this relation continues the sign and the thing signified remain united; but when it once ceases, they are no longer united; by which we are to understand, that as long as the order established by God between the sign and the thing signified remains, so long are the things exhibited and sealed with the signs..." [Zacharias Ursinus - Commentary on the Heidelberg]
"And it is this unio mystica which is signified and sealed in the Lord’s Supper." [Herman Bavinck - Reformed Dogmatics]
"The sacrament being called a communion , shows... That this ordinance is for believers only, because none else can have communion with Christ in these holy mysteries. ' Communion is based upon union .'" [Thomas Watson - The Ten Commandments]
"The dignity is amply enough commended when we hold, that it is a help by which we may be ingrafted into the body of Christ, or, already ingrafted, may be more and more united to him, until the union is completed in heaven." [John Calvin - Institutes]
VI. The Transforming Effects: Communion in Christ's Death, Resurrection, and Benefits
"Because that Christ, having redeemed and delivered us by his blood, also renews us by his Holy Spirit, after his own image; that so we may testify, by the whole of our conduct, our gratitude to God for his blessings..." [Zacharias Ursinus - Commentary on the Heidelberg]
"...by faith and the Holy Spirit we are joined to him, and also, united among ourselves just as the members are connected with the head and with each other. ... Because we are quickened and governed by Christ, and draw from him, as the fountain, all good things, so that unless we continue in him we have no life in us, as the members cut off from the body can retain no life in themselves." [Zacharias Ursinus - Commentary on the Heidelberg]
"Pious souls can derive great confidence and delight from this sacrament, as being a testimony that they form one body with Christ, so that every thing which is his they may call their own. Hence it follows, that we can confidently assure ourselves, that eternal life, of which he himself is the heir, is ours, and that the kingdom of heaven, into which he has entered, can no more be taken from us than from him..." [John Calvin - Institutes]
"Look how every bone and limb of Christ's natural body was raised up out of the grave, and carried into heaven; so shall every member of his mystical body be carried up into glory." [Thomas Watson - The Lord's Prayer]
"...when believers die, their mystical union unto Christ still remaineth, and Christ is united both unto their souls with him in glory, and to their bodies, which are his members, even when they are rotting in the grave." [Thomas Vincent - The Shorter Catechism]
"The sinner must feel his dependence on Christ in the very depths of his being, — in the sub-conscious life. Hence he is incorporated in Christ, and as a result experiences that all the grace which he receives flows from Christ. The constant feeling of dependence thus engendered, is an antidote against all self-righteousness." [Louis Berkhof - Systematic Theology]