Genesis 21:1-7

Yahweh visited Sarah as he had said, and Yahweh did to Sarah as he had spoken. Sarah conceived, and bore Abraham a son in his old age, at the set time of which God had spoken to him. Abraham called his son who was born to him, whom Sarah bore to him, Isaac. Abraham circumcised his son, Isaac, when he was eight days old, as God had commanded him. Abraham was one hundred years old when his son, Isaac, was born to him. Sarah said, “God has made me laugh. Everyone who hears will laugh with me.” She said, “Who would have said to Abraham, that Sarah would nurse children? For I have borne him a son in his old age.”

We can count on the fact that when God says something, it will happen. It may not happen the way we think and it may not happen in a way that is normal to us, but it will
happen exactly as He says. God defies the boxes we try to put Him in. Even though God isn’t in a box, He does limit Himself to His own word. That’s what it means when God tells
us that He keeps all of his promises. He will not change what He says He will do.

Titus 1:2 :

in hope of eternal life, which God, who can’t lie, promised before time began;

Some, who think they are clever, ask if God can create a rock that He cannot lift. We see here that, in a sense, He has. He will not lie, but this does not mean that God is not all powerful. It is God who makes all laws and He who decided that He won’t break them. Because the rules are self imposed, they are an example of his power, not a denial of it. In this case, God said that He would give Abraham and Sarah a son from their own bodies, and He did it even though it seemed impossible.

Isaac was the son that God had promised to Abraham many years previous to his birth. He was to be the one by whom the promises were to come. Paul even refers to Isaac as, the child according to a promise (Galatians 4:22-27). In our lives as Christians, God will keep His promise to give us the strength to do what is right. It won’t be because we help God out, God will do it in His own way even if that way seems impossible to us. God does things without our help, so that He gets the credit and we cannot act like it was because of what we have done. We must depend on His promise and only through faith in His word and trust in His power will we be able to do what is right.