We might ask then, “who’s in the Covenant of Grace?” Who are the elect of God? Can we know? We know that God revealed this promise to Abraham and his seed. In one sense it included all his’ offspring. All the male descendants were marked with the sign of the covenant which was circumcision. In another sense it only included those who were true believers. Paul tells us that only those with a circumcised heart are real believers. However we don’t know which one of our offspring will be true believers if any. So we must make a distinction between the Covenant of Grace and the Covenant of Redemption in the sense that one cannot be broken and the other can. It seems clear from history that there have been those who were included in the Covenant of Grace who turned out to be unbelievers. The same is true today. There are those who outwardly appear to belong to the church either by infant baptism or believer’s baptism but remain unbelievers and will not inherit the kingdom of God.

I think it is problematic to equate the Covenant of Grace with the outward visible church. It would be less confusing to say that the children of believers are members of "the visible church" and leave it at that. As they demonstrate a saving faith, we could then say then that they are in the Covenant of Grace (as far as we are able to discern). Since only the elect are in the Covenant of Grace, we know it cannot be broken any more than the Covenant of Redemption can.

From the Westminster Shorter Catechism
Q. 7. With whom was the covenant of grace made?
A. As the covenant of works was made with the first Adam, and all his posterity, so the covenant of grace was made with Christ, the second Adam, and in him with all the elect, as his seed, which are the Israel of God. "Now to Abraham and his seed were the promises made," (that is, not the promises of making all nations blessed.) "He saith not, Unto seeds; as of many but as of one, To thy seed, which is Christ."— Gal. 3:16. "This is the covenant that I will make with the house of Israel."— Heb. 8:10.