God promises to give a son to an old man. Abraham did not grow weak in faith. He saw that his body was already dead, yet he did not look at the weakness of the body, but at the power of the One who promised.
From bodies that were as if dead he wondrously received a son.
After he had received him, when he was commanded to offer his son as a sacrifice, he had heard the words, "In Isaac shall your seed be called" (Gen 21:12).
Yet he all but offered his only-begotten son to God, being certain that God was able even to raise him from the dead (Heb 11:19).
He bound his son and laid him upon the wood. In his intention he had already offered him in sacrifice.
But by the goodness of God, who gave him a lamb instead of the child, he received his son back alive.
Being faithful in other matters also, Abraham was sealed as a righteous man. He received circumcision as "a seal of the faith which he had while uncircumcised" (Rom 4:11), and the promise that he would be the father of many nations.
Let us consider why Abraham is the father of many nations. He is unquestionably the father of the Jews by bodily succession.
But if we look at bodily succession, we shall be forced to declare God's saying false. For according to the flesh he is by no means the father of us all.
Rather, the pattern of his faith makes us sons of Abraham.
Just as it is unbelievable to men that someone should rise from the dead, so in the same way it is unbelievable to them that children should be born from aged persons whose bodies were dead.
But we believe when Christ is preached, that He was crucified upon the tree, died, and rose.
Therefore, by the likeness of our faith to Abraham's faith, we become his sons. Then, like him, we receive with faith the spiritual seal.
We are circumcised by the Holy Spirit in Baptism, not with circumcision of the flesh, but with circumcision of the heart.
As Jeremiah says, "Circumcise for God the hardness of your heart" (Jer 4:4). And as the Apostle says, "in the circumcision of Christ, having been buried with Him in baptism" (Col 2:11-12).