The sign of God's covenant with Abraham and his descendants was circumcision. God told Abraham: "my covenant shall be in your flesh as an everlasting pact" (Genesis 17: 13). But this circumcision of the flesh did not guarantee that people would have the power to be faithful to the covenant. So God called for a new circumcision--of the heart.
Speaking to the people, after they had failed time and again to be true to the covenant, God said: "Circumcise your hearts, therefore, and be no longer stiff-necked" (Deuteronomy 10: 16). And in what is known as the last words of Moses, we hear: "The Lord, your God, will circumcise your hearts and the hearts of your descendants, that you may love the Lord, your God, with all your heart and all your soul, and so may live" (Deuteronomy 30: 6). The prophets picked up this same promise: "For the sake of the Lord, be circumcised, remove the foreskins of your hearts..." (Jeremiah 4: 4).
The new covenant that God made with humanity when Jesus came was sealed in the flesh. The heart of Jesus was "circumcised" when it was pierced as he hung on the cross. From this open heart came the water and blood representing the sacramental life of the Church. Water (Baptism) takes away sin. Blood (Eucharist) gives a new heart.
When we receive the Eucharist--the Body and Blood, soul and divinity of Jesus Christ--we receive the Sacred Heart pierced (circumcised) for us. This new heart is joined to our hearts, transforming them and making it possible for us to live the new covenant of love that Jesus sealed in his flesh. With circumcised hearts and one with the Heart of Jesus, we can now love God and neighbor as Jesus did.