V.1-4 This is the sixth time that God has appeared or given a promise to Abraham. There has been a blank in the story of Abraham of many years. Now he is 99. When we get away from the Lord, we waste our years too. This further promise and revelation bring in the subject of circumcision. This is a picture of death to the flesh, separation and purity; complete giving up, surrender to God. To enjoy our Christian blessings, we must learn the meaning of surrender to the Lord.
V.21 The covenant (agreement) of blessing was going to be through Isaac, not Ishmael. Abraham was impatient. He could not wait for God to give him a son by Sarah his wife. So he had taken the matter into his own hands, and sorrow was the result. But God is now going to act in grace.