The New Covenant is the Oldest Covenant

"So the work of Christ is the source of all human salvation from sin: the salvation of Adam and Eve, of Noah, of Abraham, of Moses, of David, and of all God's people in every age, past, present, or future. Everyone who has ever been saved has been saved through the new covenant in Christ. Everyone who is saved receives a new heart, a heart of obedience, through the new covenant work of Christ. So though it is a new covenant, it is also the oldest, the temporal expression of the pactum salutis [the covenant of peace between the Father, Son, and Holy Spirit in eternity]."

John Frame, Systematic Theology, p. 80.