God's covenant of faith and promises thereof are first made to Christ, and then to his seed in him. This must needs be so . . . 2. God's decree of election, and his execution of that decree in all the branches of it, first have respect to Christ, and then secondarily in him to all his seed: and therefore proportionately God's covenant and promises, being one branch of this execution of God's decree, must first respect Christ, then Christ's seed. Consider well: if we look at God's decree, is not Christ as Head and Mediator first of all elected, and then his seed in him? If we look at the execution of God's decree, is not Christ first accepted as God's only beloved Son, and then his seed adopted in him? Is not Christ first justified, that is, acquitted from the guilt of all the sins of his people imputed to him, and then they justified by faith in him? Is not Christ first sanctified, filled with the Spirit, made full of grace and truth, having all fullness dwelling in him; and then all his seed sanctified in him, receiving of his Spirit, and of his fullness, even grace for grace? Is not Christ first made heir of all things, and then his seed co-heirs with him? Did not Christ as a public person first die, and then all his seed die and suffer in him? Did not Christ first rise from the dead, ascend into heaven, and sit on God's right hand; and then afterwards all that are Christ's rise again, ascend into heaven, and sit on God's right hand in and with Christ? Shall not Christ first come to judge the world, and then they that are Christ's shall judge the world with him? Why then should it be thought strange that the covenant and promises should first be made to Christ, and then in Christ to all his seed? (Francis Roberts, Mysterium & medulla Bibliorum: The Mysterie and Marrow of the Bible  76).
1 Francis Roberts (1609-1675) was one of the remarkable Puritans of the 17th century. He demonstrates an uncanny `Vosian' interface between the decrees of redemption (ordo salutis) and the history of redemption (historia salutis). The reader will notice that the one is found in the other, and vice versa as they Christocentrically converge in the Eschatological Man. Here is the gist of the remarkably perceptive observation of Roberts: what God has decreed has been lived out in history by his Only-Begotten Son and therefore belongs to those in Christ. The actualized eternal decree is first and foremost lived out in the eschatological Adam/Son of God/Son of Man; and in his history, the history of those federally "in him" by grace alone is filled to the full. They are, in fact, identified with his history as he is with theirsand that decretally as well as historically.
This quotation comes from the section of Roberts's book in which he is dealing with the Westminster Larger Catechism's comments on the Covenant of Grace, Q&A 31. Spelling and punctuation have been modernized in our version above. My thanks to Benji Swinburnson for sharing this statement with us.