According to Psalms 119:29, David asked God to be gracious to him by teaching him how to obey His Law, so he was under a covenant of grace and grace has always been an attribute of God that he has shown to people throughout the OT and NT. Likewise, in Titus 2:11-14, our salvation involves being trained to do things that are in accordance with His Law, so the covenant of grace is God saying "I will teach to obey my Law".
Yes, I agree that the Covenant of Grace run from the OT to the NT. As does the Covenant of Works. Because even the OT believers where saved by Christ through Faith.
The Law was not Salvation for them. The Law held them captives, they were imprisoned under the Law. Why, were they imprisoned and held captives? Paul says in Galatians 3:For if a law had been given that could give life, then righteousness would indeed be by the law.
The Law continues to exposes their sins, to remind them of their plight before God. And their need of God's mercy.
John 5:46 For if you believed Moses, you would believe me; for he wrote of me.
Christ is the only One, that can restore the broken Covenant of Works. With perfect obedience, that God demands.
Does this mean that the Covenant of Grace (Promises of God) annul the Covenant of Works (God's Commandments)? On the contrary it fulfills it. This is the Gospel of Paul that Christ gave to him to preach.
Here is Paul on the Law & Gospel preaching in Galatians 3:
The Law and the Promise
15To give a human example, brothers:
f even with a man-made covenant, no one annuls it or adds to it once it has been ratified.
16Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ.
17This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void.
18For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise.
19Why then the law? It was added because of transgressions, until the offspring should come to whom the promise had been made, and it was put in place through angels by an intermediary.
20Now an intermediary implies more than one, but God is one.
21Is the law then contrary to the promises of God? Certainly not! For if a law had been given that could give life, then righteousness would indeed be by the law.
22But the Scripture imprisoned everything under sin, so that the promise by faith in Jesus Christ might be given to those who believe.
23Now before faith came, we were held captive under the law, imprisoned until the coming faith would be revealed.
24So then, the law was our guardian until Christ came, in order that we might be justified by faith.
25But now that faith has come, we are no longer under a guardian,
26for in Christ Jesus you are all sons of God, through faith.
27For as many of you as were baptized into Christ have put on Christ.
28There is neither Jew nor Greek, there is neither slave
g nor free, there is no male and female, for you are all one in Christ Jesus.
29And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.
Hope this helps.
In Christ our Covenantal King & Savior
