God repeatedly promised Abraham to give him very many offspring. And He made a covenant with him.
The promise to Abraham and Sarah to give them a son of their own flesh and blood was fulfilled in Genesis 21.
Isaac - which means: he laughs - was born.
God said:"It is through Isaac that your offspring will be reckoned",
and: "But My covenant I will establish with Isaac".
Genesis 17 verse 21.
The Messiah would be a descendant of Isaac.
At Isaac’s birth Sarah said: "God has brought me laughter, and everyone who hears about this will laugh with me".
God made the birth of her son possible despite her barrenness.
Hebrews 11 verse 11:
And by faith even Sarah, who was past childbearing age, was enabled to bear children because she considered Him faithful who had made the promise.
Galatians 4 verse 28 makes clear that Christians are, like Isaac, children of the promise, because the blessing of Abraham comes to them through Christ Jesus.
Galatians 3 verse 14.
Galatians 3 verse 29:
If you belong to Christ, then you are Abraham’s seed, and heirs according to the promise.