The promise was sufficient & efficacious
The Baptists believed that no covenant preceding the New Covenant was the Covenant of Grace. Before the arrival of the New Covenant, the Covenant of Grace was at the stage of promise…
If we are talking about a promise, this implies that it was not yet accomplished and was not yet in the form of a testament or a covenant. The Baptists believed that the New Covenant was the accomplishment of the promise, or in other words, the accomplishment of the Covenant of Grace. This doctrine is expressed in the following way in the 1689: “This covenant is revealed in the gospel; first of all to Adam […] and afterwards by farther steps, until the full discovery thereof was completed in the New Testament.” The New Testament brings the full revelation of the Covenant of Grace since the New Covenant is its accomplishment. The Baptists considered that the New Covenant and it alone was the Covenant of Grace.
If the New Covenant did not exist before Jesus Christ, while the Covenant of Grace existed before the coming of the Messiah, does this not mean that both covenants are distinct? The New Covenant did not exist as a covenant before Jesus Christ; however it did exist as a promise (cf. Jr 31.31). The Covenant of Grace revealed to Adam, then to Abraham, was the New Covenant promised. Therefore, before Jesus Christ, the New Covenant did not exist, but the Covenant of Grace, did not exist as a formal covenant either. John Spilsbury affirmed this notion: “Again, it’s called the promise, and not the Covenant; and we know that every promise is not a covenant: there being a large difference between a promise and a covenant. Spilsbury speaks of the Covenant of Grace that God revealed to Abraham and he declares that at this stage, it was not yet a formal covenant, but a promise. This distinction: (revealed/ concluded) summarized the difference between the Covenant of Grace in the Old Testament and the Covenant of Grace in the New Testament. In the Old, it was revealed, in the New, it was concluded [established] (fully revealed according to the expression of the 1689)…
The Abrahamic Covenant, the Sinaitic Covenant and the Davidic Covenant were not the Covenant of Grace, nor administrations of it; however, the Covenant of Grace was revealed under these various covenants.
Denault, Pascal (2014-12-13). The Distinctiveness of Baptist Covenant Theology: A Comparison Between Seventeenth-Century Particular Baptist and Paedobaptist Federalism (Kindle Locations 1195-1196). . Kindle Edition.
According to 1689 Federalism, Adam, Abraham, and Moses were all saved through their membership in the New Covenant. The typical response from paedobaptists is that we have simply manufactured this artificial distinction between the covenant of grace promised and the covenant of grace established in order to defend our view of baptism. They argue that we do not understand the relationship of a promise to a covenant.
However, note what Louis Berkhof says about the covenant of grace in Genesis 3:
1. The first revelation of the covenant. The first revelation of the covenant is found in the protevangel, Gen. 3:15. Some deny that this has any reference to the covenant; and it certainly does not refer to any formal establishment of a covenant. The revelation of such an establishment could only follow after the covenant idea had been developed in history. At the same time Gen. 3:15 certainly contains a revelation of the essence of the covenant…
Up to the time of Abraham there was no formal establishment of the covenant of grace. While Gen. 3:15 already contains the elements of this covenant, it does not record a formal transaction by which the covenant was established. It does not even speak explicitly of a covenant. The establishment of the covenant with Abraham marked the beginning of an institutional Church.
Excerpt From: Louis Berkhof. “Systematic Theology.” iBooks.
Gen 3:15 was not the formal establishment of the covenant of grace. Its formal establishment was to come later. However, it revealed the covenant of grace, and this revelation of the covenant of grace (the promise) was sufficient to save believers prior to its formal establishment.
The Westminster Confession (7.5) and Catechism (WLC 34) also teach that the promise was sufficient to save:
Q. 34. How was the covenant of grace administered under the Old Testament?
A. The covenant of grace was administered under the Old Testament by promises… [that] were sufficient for that time to build up the faith of the elect in the promised Messiah, by whom they had full remission of their sins and eternal salvation.
So there is no legitimate objection to 1689 Federalism’s understanding of the New Covenant (Covenant of Grace) as promised/established.
Furthermore, note that Berkhof says that the formal establishment of the covenant of grace marked the beginning of an institutional Church. We agree. We simply believe he was mistaken as to when the covenant of grace was formally established. Scripture is clear that it was established at the death of its mediator, Jesus Christ.
Note also LBCF 20.1
1._____ The covenant of works being broken by sin, and made unprofitable unto life, God was pleased to give forth the promise of Christ, the seed of the woman, as the means of calling the elect, and begetting in them faith and repentance; in this promise the gospel, as to the substance of it, was revealed, and [is] therein effectual for the conversion and salvation of sinners.
( Genesis 3:15; Revelation 13:8 )
Saved by the New Covenant
“When we speak of the “new covenant,” we do not intend the covenant of grace absolutely, as though it were not before in existence and effect… But for two reasons, it is not expressly called a covenant, without respect to any other things, nor was it called a covenant under the old testament. When God renewed the promise of it to Abraham, he is said to make a covenant with him; and he did so, but this covenant with Abraham was with respect to other things, especially the proceeding of the promised Seed from his loins. But absolutely, under the old testament, the covenant of grace consisted only in a promise; and as such only is proposed in the Scripture…
The greatest and utmost mercies that God ever intended to communicate unto the church, and to bless it withal, were enclosed in the new covenant. Nor doth the efficacy of the mediation of Christ extend itself beyond the verge and compass thereof; for he is only the mediator and surety of this covenant.”
-Owen (Commentary, Hebrews 8:6)
“There is yet no reason why God should not have extended the grace of the new covenant to the fathers. This is the true solution of the question.”
-Calvin (Commentary Hebrews 8:10)
“And so it finally came to the most perfect promise of all, that of the new testament, in which, with plain words, life and salvation are freely promised, and actually granted to those who believe the promise. And he distinguishes this testament from the old one by a particular mark when he calls it the “new testament” [Luke 22:20; I Cor. 11:25]. For the old testament given through Moses was not a promise of forgiveness of sins or of eternal things, but of temporal things, namely, of the land of Canaan, by which no man was renewed in spirit to lay hold on the heavenly inheritance.”
-Luther (The Babylonian Captivity of the Church)
“As then the law of works, which was written on the tables of stone, and its reward, the land of promise, which the house of the carnal Israel after their liberation from Egypt received, belonged to the old testament [covenant], so the law of faith, written on the heart, and its reward, the beatific vision which the house of the spiritual Israel, when delivered from the present world, shall perceive, belong to the new testament [covenant]… I beg of you, however, carefully to observe, as far as you can, what I am endeavouring to prove with so much effort. When the prophet promised a new covenant, not according to the covenant which had been formerly made with the people of Israel when liberated from Egypt, he said nothing about a change in the sacrifices or any sacred ordinances, although such change, too, was without doubt to follow, as we see in fact that it did follow, even as the same prophetic scripture testifies in many other passages; but he simply called attention to this difference, that God would impress His laws on the mind of those who belonged to this covenant, and would write them in their hearts, (Jer 31:32-33)… These pertain to the new testament [covenant], are the children of promise, and are regenerated by God the Father and a free mother. Of this kind were all the righteous men of old, and Moses himself, the minister of the old testament, the heir of the new,—because of the faith whereby we live, of one and the same they lived, believing the incarnation, passion, and resurrection of Christ as future, which we believe as already accomplished”
-Augustine (Treatise on the Spirit and the Letter, c. 41, 42; A Treatise Against Two Letters of the Pelagians, b.3 c. 11)