In his Institutes of the Christian Religion, John Calvin teaches that “baptism is the initiatory sign by which we are admitted to the fellowship of the Church, that being ingrafted into Christ we may be accounted children of God” (book 4, chapter 15.1). He goes on to explain from Scripture that there are two main reasons that God gave the church the sign of baptism: the first reason is to strengthen our faith in him; the second reason is to confess our faith among men.
How does baptism strengthen our faith? Calvin gives three ways. First, God gives us baptism as a visual sign and promise of our purification from sin, a washing “by which he assures us that all our sins are so deleted, covered, and effaced, that they will never come into his sight, never be mentioned, never imputed.” The record of our sin is erased, and sin will never again be recorded against us.
Second, not only is the record of sin deleted, but the power of sin in our lives is broken. Baptism shows us our mortification in Christ (death to sin) and our new life in him. The waters of baptism are the closest we will ever come to having the wine of God’s wrath poured out upon us, because it was poured out on Christ on behalf of his people. He died for us, and we identify with him and his death by baptism. We are to consider ourselves as dead to sin. Just us the water washes the body outwardly, the Holy Spirit according to Titus 3:5-7 washes us spiritually and inwardly, and so we have the power of resurrection life already at work within us.
Third, baptism strengthens our faith by assuring us visibly of an invisible truth: that we are ingrafted into Christ (to use Paul’s language of Romans 11), or joined to him, united to him, and therefore recipients of all his benefits, both in the present and the future: “For he consecrated and sanctified baptism in his own body, that he might have it in common with us as the firmest bond of union and fellowship which he deigned to form with us; and hence Paul proves us to be the sons of God, from the fact that we put on Christ in baptism.” As Paul says in Galatians 3:27-29, “As many of you as were baptized into Christ have put on Christ … and if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.”
To summarize, baptism strengthens our faith by portraying for us that the record of sin is obliterated, the power of sin in our present and future is broken, and through union with Christ we have all the resources of God in heaven at work within us to bring us to our glorious inheritance. All of this is the reality—accomplished by God alone, by his grace, in his power, and for his glory—that the sign of baptism points us to and promises us. All of that is from God, but he has something in mind for us to do, too.
The other main reason God gave us baptism is to confess our faith before the world: “Baptism serves as our confession before men, in as much as it is a mark by which we openly declare that we wish to be ranked among the people of God, by which we testify that we concur with all Christians in the worship of one God, and in one religion; by which, in short, we publicly assert our faith, so that not only do our hearts breathe, but our tongues also, and all the members of our body, in every way they can, proclaim the praise of God. In this way, as is meet, everything we have is made subservient to the glory of God, which ought everywhere to be displayed, and others are stimulated by our example to the same course.” In baptism we are identifying ourselves with the name of Christ before men, but most importantly, Christ is laying hold of us and making us one with himself.
Baptism is a sign and seal given to the church by God, so when the church administers or receives the sacrament, it is God himself acting through us as his agents. Since all the spiritual benefits represented in baptism are gifts of God, and since baptism is also given to us as an ordinance in the church by God, we should think of baptism as being administered by God. He is the one, through his appointed agents using his appointed sign, who signifies and seals Christ and all his benefits to the one baptized: “For inasmuch as [baptism] is appointed to elevate, nourish, and confirm our faith, we are to receive it as from the hand of its author, being firmly persuaded that it is himself who speaks to us by means of the sign; that it is himself who washes and purifies us, and effaces the remembrance of our faults; that it is himself who makes us the partakers of his death, destroys the kingdom of Satan, subdues the power of concupiscence, nay, makes us one with himself, that being clothed with him we may be accounted the children of God.”