1) Covenant theology states that there was a covenant made first between the Father and the Son. This was a pre-temporal covenant. You may recall that in some of my sermons I have referred to the "Pactum salutis." The Father gave to the Son the elect to save. This He did at the cross.
2) This eternal covenant stands behind the several historical covenants. While there are many covenants, ultimately there is but one promise (Eph 2:12). This was first made with Abraham (really to Adam and Eve in the garden) before he was circumcised (recall the early sermons that I did in Galatians 3) and so the covenant made with the nation of Israel was a temporary and external covenant until the Seed should come.
3) The Old Covenant (OC) anticipates the New at several places. Why? Because the promises were made to Abraham and his Seed, that is Christ and all those and only those that are united to Him by faith alone.
4) When the OC anticipates the NC it speaks of it as a better covenant (Jer 31:31-34; Heb 7:22; 8:6) What makes this covenant better than the first? Here is the Jeremiah passages:
Jer 31:31 "Behold, days are coming," declares the LORD," when I will make a new covenant with the house of Israel and with the house of Judah, not like the covenant which I made with their fathers in the day I took them by the hand to bring them out of the land of Egypt, My covenant which they broke, although I was a husband to them, declares the LORD. But this is the covenant which I will make with the house of Israel after those days, declares the LORD, "I will put My law within them and on their heart I will write it; and I will be their God, and they shall be My people. They will not teach again, each man his neighbor and each man his brother, saying, 'Know the LORD,' for they will all know Me, from the least of them to the greatest of them," declares the LORD, for I will forgive their iniquity, and their sin I will remember no more."
Note:
a. this covenant is *not like* the previous covenant. There is dis-continuity.
b. God writes the law, that is all 10 commandments on the heart of all those in the NC. This is part of regeneration (Eze 11:19-20; 36:25-27).
c. there will be covenant oneness: "I will be their God, and they shall be My people" All those in the NC will have God as their God. Again, recall the elect given by the Father and then promised to Abraham (please see Romans 9:6-16 with John 1:12-13) This covenant oneness entails:
i. there is no need of teachers in the NC. That is, there is no need for those that would instruct others to "know the LORD", because every (from the least to the greatest) NC believer *will* know God (John 17:3).
ii. all NC people have their sins are forgiven.
How does baptism relate. Here is what the WMC (a paedobaptist confession) says of baptism:
Baptism is a sacrament of the new testament, ordained by Jesus Christ, not only for the solemn admission of the party baptized into the visible church; but also, to be unto him a sign and seal of the covenant of grace, of his ingrafting into Christ, of regeneration, of remission of sins, and of his giving up unto God, through
Jesus Christ, to walk in newness of life. Which sacrament is, by Christ's own appointment, to be continued in his church until the end of the world.
To this (except the language of sealing) Reformed Baptist say AMEN! Notice that this statement blends perfectly with what is said about the NC. Regeneration (a new heart). Remission of sins. Engrafting into Christ. Newness of life. These are all true of NC believers. Full stop. At this point the WMC and the LBC are in full agreement.
The mark (baptism) of the NC belongs only to those that share in the above realities. So, unless one is willing to say (like Rome) that baptism communicates these graces, baptism is for the elect sons (seed) of Abraham the believer.