"The Abrahamic covenant referred to blessings that were YET FUTURE (e.g.. the justification of "all the nations" (Gal 3:8) through faith in Christ. The New Covenant on the other hand speaks of these same blessings NOW REALIZED "in the fullness of time."
Scripture foresaw that God would justify the Gentiles by faith, and announced the gospel in advance to Abraham: "All nations will be blessed through you." Gal 3:8
Paul describes the events surrounding the coming of Christ and the inauguration of the New Covenant by the term "Faith". He does this in order to contrast the gospel's central principle (faith) with the Old Covenant's central principle (works).
With the coming of Christ, Promise finds its culmination and fulfillment. "All the promises of God find their 'Yes' in Him." Those promises include Christ's death and resurrection for the salvation of His people, the ingathering of the Gentiles and their justification by faith, personal "teaching" from God for every true child of Abraham, and especially the "promised" gift of the Holy Spirit.
With the coming of Christ, Law also finds its culmination and fulfillment. Christ fulfills every one of the Law's prophesies, sacrifices, types, and shadows. He also fulfills every righteous principle of moral conduct embodied in the Law. Not only this, but Christ fulfills all the legal "conditions" of the Mosaic Law represented by its blessings and curses. By His perfect life, He satisfies on our behalf the Law's positive demands, and by His perfect death, He satisfies on our behalf the Law's negative demands.
Both Promise and Law find their culmination and fulfillment in the glorious Person and work of Christ. Promise is swallowed up as anticipation gives way to realization. and Law is swallowed up as demand gives way to satisfaction.
From The Law of Christ by Charles Leiter
Note: While I do not agree with everything in this book, most of it is clear, intelligent and biblical.