For the Particular Baptists, all of the Old Testament saints were concurrently members of both the Old and the New Covenant.
As members of the Covenant of Circumcision, Old Testament Saints (like Abraham) were heirs to the physical land promises that God gave to Abraham, Isaac, and Jacob. As members of the New Covenant, these Old Testament saints were saved by none other than the life, death, resurrection, imputed righteousness, and finished works of Jesus Christ. Those who looked past the types and shadows of the Old Covenant to the antitype of those things that is Christ, were saved by His blood shed for the sins of His people at the cross.
The 1689 London Baptist Confession of Faith explains this with clarity…
“This covenant is revealed in the gospel. It was revealed first of all to Adam in the promise of salvation through the seed of the woman. After that, it was revealed step by step until the full revelation of it was completed in the New Testament. This covenant is based on the eternal covenant transaction between the Father and the Son concerning the redemption of the elect. Only through the grace of this covenant have those saved from among the descendants of fallen Adam obtained life and blessed immortality. Humanity is now utterly incapable of being accepted by God on the same terms on which Adam was accepted in his state of innocence.”
~ 1689 LBC 7.3
This is an important distinction that continues today between the Particular Baptists and their paedobaptist brethren, such as the Presbyterian and Republication types. But it also is an important distinction between those who subscribe to what is known as 1689 Federalism, and those that are known as Reformed Baptists of today that do not.
For the Particular Baptists, only the New Covenant is the Covenant of Grace. Those who disagree with that can still subscribe to the 1689 London Baptist Confession of Faith, for these nuanced but important differences between us are allowed within the Confession itself.
Knowing these differences can help Particular Baptists moving forward within their own tradition, as well as provide clarity for our Presbyterian and Republication brethren who wish to “engage” with our tradition. In other words, it will allow them to know what they’re arguing against. It also will prevent the “I used to be like you” argument, that usually results in a realization two hours of Twitter engagement later that you never were really like me in the first place.
You can find more information concerning the 1689 Federalism position at 1689federalism.com, and learn more about the unity & diversity of the Reformed tradition, the roots of the Particular Baptists who came from within that unity & diversity, as well as their view of Gods covenants here, and here.