Consider the promise in Gen 17:

17 When Abram was ninety-nine years old, the Lord appeared to him and said, “I am El-Shaddai—‘God Almighty.’ Serve me faithfully and live a blameless life. 2 I will make a covenant with you, by which I will guarantee to give you countless descendants.”

7 “I will confirm my covenant with you and your descendants[b] after you, from generation to generation. This is the everlasting covenant: I will always be your God and the God of your descendants after you.

9 Then God said to Abraham, “Your responsibility is to obey the terms of the covenant. You and all your descendants have this continual responsibility. 10 This is the covenant that you and your descendants must keep: Each male among you must be circumcised. 11 You must cut off the flesh of your foreskin as a sign of the covenant between me and you. 12 From generation to generation, every male child must be circumcised on the eighth day after his birth. This applies not only to members of your family but also to the servants born in your household and the foreign-born servants whom you have purchased. 13 All must be circumcised. Your bodies will bear the mark of my everlasting covenant. 14 Any male who fails to be circumcised will be cut off from the covenant family for breaking the covenant.”

2 Cor 1:20 For all the promises of God in him are yea, and in him Amen, unto the glory of God by us.