"When God made his promise to Abraham, since there was no one greater for him to swear by, he swore by himself, saying, “I will surely bless you and give you many descendants.” And so after waiting patiently, Abraham received what was promised" (Hebrews 6:13-15)
I remember going to Sunday school as a child and learning about Abraham. We learned about the birth of Isaac and how it came about because God promised him a son. I remember a teacher bringing a jar of sand to the class and asking us to count it. Of course we couldn't but the object lesson rang true, Abraham was blessed by God and his descendants were going to be numerous.
Since that time, I think often about Abraham and how he waited a long time to see this promise fulfilled. He was one hundred years old, Sarah was ninety yet he considered the one who promised him, faithful. That is why he held to the hope of having a son and also saw fulfillment of it.
The amazing thing about all of this is that Isaac was really just the down payment of the promise. Even the great number of people descending from Abraham was merely one of the blessings that came along with the promise. The real implication of this promise was that through the line of Abraham and Isaac the Messiah would come (Galatians 3:8-9, 16, Genesis 12:3; 13:15; 17:8). Galatians makes this very clear, “The promises were spoken to Abraham and to his seed. The Scripture does not say “and to seeds,” meaning many people, but “and to your seed,” meaning one person, who is Christ” (Galatians 3:16). This was a great promise and with it God announced the gospel in advance to Abraham when He made this pledge (Galatians 3:8).
God promised this to Abraham with an oath. In court, when giving testimony, the witness is sworn in with the Bible, the promise is that they will tell the truth and the object used is the guarantor of the promise. Scripture says God swore by himself since there was nothing greater to guarantee His promise to Abraham (Hebrews 6:13). This pledge was backed by His namesake, His word and His power. This means that it would be fulfilled at all cost. Abraham may have gotten the down payment with Isaac, but we have seen fulfillment in Jesus.
The great thing about Abraham is that he believed God and it was credited to him as righteousness (Genesis 15:6; Romans 4:3; Galatians 3:6; James 2:23). Scripture mentions this four times which means it is incredibly important for us. He took God’s guaranteed promise, backed by an oath, and knew that it would come to pass. He believed God and was justified by faith the same way we are today. “This is why “it was credited to him as righteousness.” The words “it was credited to him” were written not for him alone, but also for us, to whom God will credit righteousness—for us who believe in him who raised Jesus our Lord from the dead” (Romans 4:22-24). Abraham is called the father of faith for a reason. His faith has truly blessed the world as it was the conduit for the promise of God to come to pass.