Coxe on New Covenant Union with Christ

[T]he covenant of grace is established in Christ as its head. All its promises were first given to him and in him they are all yes and amen. It is by union to him that believers obtain a new covenant interest and from him they derive a new life, grace, and strength to answer the ends of the new covenant. (40)

[A]ll the blessings of this covenant redound on believers by means of their union and communion with the Lord Jesus Christ, who is both the Head and Root of the new covenant, and the Fountain from which all its blessings are derived to us. Since these blessings were entirely purchased by him, so are they entirely applied to all that are in him and to none other… [N]one are at any time justified before God except those whom Christ has loved and washed from their sins in his own blood (Revelation 1:5). None are washed by him but those that are in him as the second Adam. It is by union to him as the root of the new covenant that the free gift comes on them to the justification of life (Rom 5:14ff). And none can have union to him but by the indwelling of his Holy Spirit. Wherever the Spirit of God applies the blood of Christ for the remission of sins he does it also for the purging of the conscience from dead works to serve the living God. As certainly as any derive a new covenant right from Christ for pardon, they also receive a vital influence from him for the renovation of their natures and conforming their souls to his own image. (81-82)

The grace and blessings of the new covenant were given and ensured to Abraham for himself. (75)

During the time of the law… [t]he children of God after the Spirit (though as underage children they were subject to the pedagogy of the law, yet) as to their spiritual and eternal state, walked before God and found acceptance with him on terms of the covenant of grace… this spiritual relationship to God [was] according to the terms of the new covenant which the truly godly then had… (133)

–Covenant Theology from Adam to Christ