God did from all eternity decree to justify all the elect, and Christ did in the fullness of time die for their sins, and rise again for their justification: nevertheless, they are not justified personally, until the Holy Spirit doth in due time actually apply Christ unto them. (With Savoy Declaration of Faith and 2nd London Baptist Confession of Faith’s edits of Westminster Confession Chapter XI)
The Triune God must be worshiped through his works and the effects of his works. That is the great and glorious end of the receiving this great salvation (Heb 2:3). However, how do you partake in this telos (purpose) of eternally communing with the Triune God and rightfully confess the benefit of worshipping God as Father? Through the one who has the essential image of the Father in his glorious Person through eternal generation: Jesus (Heb 1:3; Col 1:15). The glorious Person of Christ, set with offices and a work to perfectly accomplish by the federal covenant he was under (Luke 22:29, John 17, Hebrews 7:22), was for those people that will behold the glory of God in the face of Jesus Christ (2 Cor 4:4-6) by walking by faith now and sight in the future (2 Cor 5:7). To partake in this requires faith alone in Christ. Salvation’s basis is the whole Lord Jesus Christ for his people in the Spirit.
Great Theologians of the past have emphasized this through the true and mandatory doctrine of Union with Christ. The Westminster Confession, Savoy Declaration, and 2nd London Baptist all recognize three points in time: First, God’s decree. Second, Christ’s satisfaction. Finally, the regeneration and faith of God’s people. Reformed Theology distinguishes this as a Decreed Union flowing from God’s decree (Eph 1:4), a Federal Union (Rom 6:6; Gal 2:20), and a vital or Mystical Union (1 Cor 1:9; 6:17). This wonderfully explains the substance of how the Triune God gives the salvation purchased by Christ in his death and resurrection, which he applies to ungodly sinners, he federally represented by an application of the life-giving Spirit.
However, what Scripture reveals explains this even further. The fulfillment of the covenant that Christ was under was for the purpose of applying the benefits of salvation to those whom the covenant ascribes to (Hebrews 7:22). John Owen succinctly explains this in his treatise on Justification by Faith:
The covenant may be considered as unto the actual application of the grace, benefits, and privileges of it unto any personal whereby they are made real partakers of them, or are taken into covenant with God; and this alone, in the Scripture, is intended by God’s making a covenant with any… He thereby underwent and performed all that which, in the righteousness and wisdom of God, was required; that the effects, fruits, benefits, and grace, intended, designed, and prepared in the new covenant, might be effectually accomplished and communicated unto sinners. (Works V: 253)
When God makes a covenant with his Church (Hebrews 8), he is applying Christ absolutely through union with Christ in the Spirit. This is then our mystical union with Christ which is the same as our federal union with Christ. Christ is only a federal representative of his people until his people by union with Christ by the effectual call inaugurates Christ as Head of his People. However, God will always be faithful to his covenant and this is proven through our Federal Union with Christ into which his people were called through the Spirit. As the Apostle Paul explains: “God is faithful, by whom you were called into the fellowship of his Son, Jesus Christ our Lord.” (1 Cor 1:9).
If salvation is through the Triune God in Christ, you must participate in Christ. To participate in Christ, you must be united to Christ. Thus, I participate in Christ when I am united to Christ. This even happens before we believe, because the effectual call of God establishes this union through God sovereignly making a covenant of free grace with his people (Westminster Shorter Catechism Q. 30). Behold the faithfulness of God through his sovereign provision of every single benefit and promise of salvation to the Church through the faithful husband Jesus Christ!
This is a reminder that everything needed for salvation is literally in Christ alone. To all those living and walking by faith, the Triune God has proven his faithfulness to you. So let us sing with our hearts, all glory be to Christ!