Baptism – Sign of the Covenant
Once we come to Christ, we then have the privilege of participating in water baptism which is an outward sign of our relationship with God through Christ.  It is (according to Romans 6) a means of identifying with his death, burial, and resurrection.  You may also wish to think of baptism as a prophetic fulfillment of the Old Testament sign of circumcision.  In the Old Testament circumcision was called the “sign of the covenant”.  It was used in the same sense that Christians view the Lord’s Supper and baptism — an outward sign of participating in the dual aspects of the Abrahamic covenant (promise and covenant respectively).

Colossians makes this connection between circumcision and baptism.

…Christ performed a spiritual circumcision—the cutting away of your sinful nature.  For you were buried with Christ when you were baptized.
Colossians 2:11-12

But for the ancient Israeli community, it became obvious that there was a difference between outward and inward circumcision.

• In Leviticus, God talks about Israel’s future treachery and hostility towards him, using the term “uncircumcised hearts” (Leviticus 26:40-41).

• Moses commands the people: “Circumcise your hearts, therefore, and do not be stiff-necked any longer” (Deuteronomy 10:16).

• At the giving of the Law, Moses says, “The Lord your God will circumcise your hearts and the hearts of your descendants, so that you may love him with all your heart and with all your soul, and live” (Deuteronomy 30:6).

• Jeremiah called on the Israelites to “Circumcise yourselves to the Lord, circumcise your hearts….” (Jeremiah 4:4) and mourned that “the whole house of Israel is uncircumcised in heart” (Jeremiah 9:26).

• Stephen angered his enemies by calling them “stiff-necked people, with uncircumcised hearts and ears!” (Acts 7:51, referring to Deuteronomy 10:16).

• Paul wrote, “A man is a Jew if he is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code” (Romans 2:29).

In these passages we discern that physical circumcision was not enough; the physical act of cutting away foreskin had no effect on the wicked heart.  Therefore, an internal work was needed to purify the heart of man.

On the day of Pentecost, Peter replied, “Each of you must repent of your sins and turn to God and be baptized in the name of Jesus Christ for the forgiveness of your sins. Then you will receive the gift of the Holy Spirit.  This promise is to you, to your children, and to those far away – all who have been called by the Lord our God.”
Acts 2:38–39

In the New Testament, we find both water baptism and a baptism or filling with the Spirit of God.  The first functions as a means of identifying with the death, burial, and resurrection of Jesus as well as a testimony to the world that we belong to Christ.  The second is a means of addressing the ‘circumcision of the heart’ mentioned earlier.  As we invite him the Spirit of God moves in and begins an internal circumcision of the heart, thought life, attitudes, etc.  This is the fulfillment of that which Moses and Jeremiah prophetically called “circumcision of the heart” and today is the privilege of every believer who yearns for the fullness of his Spirit.