Genesis 21:4 And Abraham circumcised his son Isaac being eight days old, as God had commanded him.
God made covenant promises to Abraham out of sheer grace. Abraham received the Covenant and believed the Promise. Because of grace he obeyed God’s command, after faith was engendered in his heart. God had commanded circumcision for him and all the males in his household. Now he responded to God’s impossible gift of a son in his old age. He was 100 years and Sarah was 90 and post-menopausal. He waited and believed for 25 years. Who wouldn’t respond with obedience after such a miracle fulfillment?
Our Lord made a covenant with us in Holy Baptism and commanded us in Holy Communion to ”Do this.” Circumcision is a foreshadowing of the New Covenant Sacraments. We respond in faith and obedience by baptizing our family (at about eight days old) and participating in the Lord’s Supper (often). This is the outward,, visible Word of the covenant sealed by the blood of Jesus.
The Lord has done the same kind of impossible miracle in us by giving life to our dead-in-sin souls. This new life responds to grace by teaching and believing the covenant. We never get over, get used to, or get numbed to the wonderful grace, mercy, love, forgiveness, and salvation of God.