Maybe it has always been God’s grace?
John 1:16-17 For we have all received from his fullness one gracious gift after another. For the law was given through Moses, but grace and truth came about through Jesus Christ.
Sometimes, I think the Church falls into this idea that history splits between opposing Old Testament and New Testament dynamics.
The common view holds that the giving of the Mosaic Law marked the Old Testament, and it was a harsh era of God that doomed the Hebrews, at least for a time, under a Law they could never really fulfill.
Then, in a New Testament world, many consider that we live in the era of grace, removed from the Law of the Old Testament — history split, not just by Jesus, but by the split between a time of Law and a time of grace.
John 1:16-17 stands in opposition to such an idea. The passage offers that it has always been about God's grace. It is God's grace that gifted the Law through Moses, and God's grace that sent Jesus to deliver grace and truth. (I read grace and truth, linguistically, as a single, combined gift.)
One does not receive a gift and then give it back when they get another. Rather, a second gift is added to someone's life, in addition to the first. This means that John is telling us that God has given us one gracious gift "on top of another," and now we hold two gifts: the Law, and grace and truth.
We can then better call our Scriptures — one marked by the gracious gift of the Mosaic Law and one marked by the gracious gift of grace and truth —the First Testament and the Second Testament. One adds to the other as one gift is accumulated after another.
Calling them the Old and New Testament risks thinking we can care little for what is old/dated/expired, and we may dismiss that "old" testament.
As the gifts accumulate, including the Mosaic Law, we find it has really always been the grace of God.
Therefore, the dynamics of the Old and New Testaments do not actually stand in opposition. The First and Second Testament periods have always been about the dynamic grace of God.