“In your seed all the nations of the earth shall be blessed, because you have obeyed My voice.” Genesis 22:18
Abraham and Sarah were very old when God kept His promise and gave them a son, Isaac. God tested Abraham to see if he would choose God or his only son. Abraham trusted God and chose to obey God, even though it was hard, and God kept His promise to Abraham that the whole world would be blessed through his son.
And if you are Christ’s, then you are Abraham’s seed, and heirs according to the promise. Galatians 3:29