God’s promises defy the natural. They are not contained by our understanding. Read Galatians 4:22-27.
But the son by the bondwoman was born according to the flesh, and the son by the free woman through the promise. This is allegorically speaking, for these women are two covenants: one proceeding from Mount Sinai bearing children who are to be slaves; she is Hagar. Now this Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free; she is our mother. For it is written, ” Rejoice, barren woman who does not bear ; Break forth and shout, you who are not in labor ; For more numerous are the children of the desolate Than of the one who has a husband .”
Galatians 4:23-27 NASB
As Paul continues discussing the symbolic understanding of the 2 wives and 2 sons of Abraham, we see that when God makes the promise, he is not bound by the natural order of the world that he created. God is more and is above his creation and he can work and move in any way that he likes. Just because Sarah was not able to have children didn’t mean that she wasn’t able to be the mother that fulfilled God’s promise to Abraham.
God was able to work and move in Sarah’s life and to allow her to have a child and through that child, to have a multitude of other children. As we are followers of Jesus Christ, we also become children of Abraham through the promise.
You should never under estimate or short change what God can do. God is always able to fulfill his promises, generally in bigger and greater ways than we could ever possibly imagine. Are we following after him no matter what? Do we seek him even when we don’t understand, because he is always able to fulfill his promises.