In Reformed Baptist Covenant Theology and Biblical Theology (found in the volume Recovering a Covenantal Heritage), Micah and Samuel Renihan explain New Covenant union with Christ.
Jesus Christ has been and always will be the federal head of the covenant of grace/New Covenant. To be federally united to him you must be 1) promised to him outside of time in the covenant of redemption and 2) brought into union with him in time by the Holy Spirit.
The Son was the one elected by the Father to win the redemption of the elect. All of this is accomplished in the New Covenant, which is the historical climax of the covenant of grace. To be in the covenant of grace/New Covenant, you must be united to Christ, its federal head.17
Since the covenant of grace is the retro-active application of the New Covenant, if we posit that Christ is the mediator of the covenant of grace, we can only understand the terms of his role as mediator, and our relation to him as such, through the way that he is presented in the New Covenant. That Christ is the mediator of the covenant of grace, the New Covenant, no Reformed theologian denies. Thus, in line with New Testament doctrine, the only way to be under Christ’s federal headship is to be united to him by the Holy Spirit. This union finds its roots outside of time as we are chosen in Christ in the covenant of redemption and is applied to the elect in time by the Spirit, begun in effectual calling and consummated in the faith of the believer. Apart from saving faith there can be no union with Christ, because the Spirit does not indwell any except the elect, those who have been justified by faith.18 Christ is the one and only federal head of the covenant of grace, the New Covenant. Federal headship is never mediate, thus none can enter the covenant other than those who are directly or immediately under his federal headship by the Holy Spirit.19
17 Cf. WLC 57-59.
18 Cf. Acts 2:38, Eph. 1:13.
19 Cf. WLC 65-69.
To bring this to a conclusion, a right understanding of the membership of the covenant of grace is founded on the covenant of redemption and the New Covenant. Those who are in the covenant of grace are those who were promised to the Son by the Father in the covenant of redemption, won by the Son’s life, death, and resurrection, and sealed by the Holy Spirit, uniting them to their federal head, Jesus Christ. Laying claim to Christ and his benefits is a serious matter, and as Scripture shows, only those who have saving faith can truly make that claim. There is no external federal relation to Jesus Christ. In terms of membership or qualification, there are no distinctions in the body of Christ, that is, the church. All are sons of God through faith, under one head, indwelt by one Spirit. “Anyone who does not have the Spirit of Christ does not belong to him” (Rom. 8:9). In spite of the false professions, unbelief, and lies of apostates, God knows his own, Christ knows his sheep, and the Spirit of adoption knows the children of God.26 The covenant people of God are “a chosen race, a royal priesthood, a holy nation, a people for his own possession” (1 Pet. 2:9). The glorious New Covenant does not look to the Old for its pattern and people but stands on the eternal foundation of the covenant of redemption and comes to the elect as a covenant of grace, purchased, mediated, and eternally kept by “our great God and savior Jesus Christ who gave himself for us to redeem us from all lawlessness and to purify for himself a people” (Titus 2:14).
26 Cf. 2 Tim. 2:19, John 10:27, Matt. 7:15-23, Rom. 8:16.