Although baptism is a sacrament of the new covenant, that does not mean we understand it from only the new covenant revelation. In fact, some of the most important teaching about baptism and its significance is given in the Old Testament.
To understand how this can be the case, we must first look at Colossians 2:11–12, wherein Paul uses the language of both circumcision and baptism to refer to spiritual realities that are true for all those who are in Christ Jesus by faith alone. Because of this connection between circumcision, baptism, and particular spiritual truths, we may rightly infer that new covenant baptism corresponds to old covenant circumcision, both rites signing and sealing the same reality under different administrations of the one covenant of grace. This is the conclusion of John Calvin, who comments: “Christ . . . accomplishes in us spiritual circumcision, not through means of that ancient sign, which was in force under Moses, but by baptism. Baptism, therefore, is a sign of the thing that is presented to us, which while absent was prefigured by circumcision.”
But what was circumcision supposed to sign and seal? Paul answers this question for us in his reference to the “circumcision made without hands.” This is another way of speaking of the circumcision of the heart that the prophets proclaimed to old covenant Israel. Deuteronomy 10:12–22, for instance, calls for Israel to have circumcised hearts. Physical circumcision was a visible mark that separated the Israelites from the pagan Gentiles (Gen. 17), but in itself all it accomplished was designating those who received the signs as members of the visible covenant people. To be God’s people in the truest and fullest sense, there had to be circumcision of the heart, a separation of one’s love and commitment away from the things of this world unto the things of God. In essence, circumcision pointed to the need for regeneration under the old covenant just as baptism points to the need for regeneration under the new covenant.
Circumcision also wounds the flesh, revealing the consequences of those who broke the covenant. Those who were not circumcised in the flesh would be cut off from the visible covenant people (Gen. 17:14), corresponding to the spiritual reality that those who were not circumcised in the heart would be cut off from eternal salvation. Those who receive God’s covenant signs face consequences if they do not keep their covenant obligations.