Then the LORD your God will bring you to the land which your fathers possessed, and you shall possess it. He will prosper you and multiply you more than your fathers. And the LORD your God will circumcise your heart and the heart of your descendants, to love the LORD your God with all your heart and with all your soul, that you may live.
Deut 30:5–6
In v5, the Lord says that He will prosper and multiply His people more, after the exile, than He had before the exile. But, historically, we did not see this until the grafting in of those elect from among the nations.
It is helpful for our understanding of God's dealing with covenant children, in the administration under Moses, that His promises to them were not only visible/externally covenantal, but internal and spiritual: "YHWH your God will circumcise the heart of your offspring, to love YHWH your God with all your heart and with all your soul."
And it is helpful for our understanding of God's dealing with covenant children, under Christ, is that this "and the heart of your descendants" aspect of the promise has a focus especially on the time in which the visible church would be prospered and multiplied "more than your fathers."
When it included children, under Moses, it was the covenant of grace.
And under Christ, the covenant of grace still includes children!
When the apostle says, "For the promise is to you and your children, and and to all who are afar off, as many as the Lord our God will call," (Ac 2:39), he is saying exactly what Deut 30:5–6 says.
What is Spirit baptism, but the circumcision of the heart? It corresponds to water baptism just as the Lord's gracious work of heart-circumcision corresponded to the covenant sign that He had commanded upon covenant children.
Deut 30:5–6 was solving the problem of a people who could not circumcise their own hearts, as commanded in Deut 10:16. And, on the day of Pentecost, there was this wonderful proclamation that Jesus provides, by His Spirit, the repentance that the baptism of John could only announce to us that we need it. Christian baptism announces that Jesus actually gives it.
This hope He gives us for ourself. This hope He gives us for our children.