In Belgic Confession article 29, the Reformed Churches identify three marks of the true church and seven marks of the true Christian. The second mark of the true church says: “It makes use of the pure administration of the sacraments as Christ instituted them.”
Regarding baptism, we confess:
For that reason we detest the error of the Anabaptists who are not content with a single baptism once received and also condemn the baptism of the children of believers. We believe our children ought to be baptized and sealed with the sign of the covenant, as little children were circumcised in Israel on the basis of the same promises made to our children. And truly, Christ has shed his blood no less for washing the little children of believers than he did for adults. Therefore they ought to receive the sign and sacrament of what Christ has done for them, just as the Lord commanded in the law that by offering a lamb for them the sacrament of the suffering and death of Christ would be granted them shortly after their birth. This was the sacrament of Jesus Christ. Furthermore, baptism does for our children what circumcision did for the Jewish people. That is why Paul calls baptism the “circumcision of Christ.”
A mark of the true church—by definition—is essential, and that which is essential cannot be secondary.
Baptism is one of the two sacraments instituted by our Lord Jesus Christ. He instituted it as a sacrament in the great commission:
And Jesus came and said to them, “All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age” (Matt 28:18–20; ESV).
Baptizing is part of how we make disciples. It is and has always been basic to Christianity just as the Lord’s Supper has been. The Biblical and historical evidence is clear enough for the Reformed churches. God instituted the inclusion of believers and their children into the visible church under Abraham. That command and promise (Gen 17:7) have never been revoked. All the evidence is that the earliest Christians followed that pattern. The Apostle Peter repeated that formula in Acts 2:39, and we see households being baptized in Acts 16.
By AD 206 we have clear evidence that the post-apostolic church was baptizing infants. Both Tertullian and Origen recognized it as did Hippolytus in AD 215. Cyprian insisted on it in AD 253 and Augustine knew no other practice in the church. There is no evidence of controversy over infant baptism in the early church and were infant baptism a novelty in the early third century there would have been controversy. In that same period, the controversy over when to observe the Christian pascha (later Easter) nearly split the church. Were infant baptism a new practice we would almost certainly know about it, but we do not.
Who thinks it is a secondary doctrine?
Some people approach a wide range of issues by asking whether it is a “salvation issue.” This is one of those ostensibly clever questions that is too clever by half. What is a “salvation issue” anyway? The Trinity and the two natures of Christ were, according to the Athanasian Creed in the fifth century, “Whosoever will be saved: before all things, it is necessary that he hold the catholic faith: Which faith except everyone do keep whole and undefiled: without doubt, he shall perish everlastingly. And the catholic faith is this: That we worship one God in Trinity, and Trinity in unity…Furthermore, it is necessary to everlasting salvation: that he also believe rightly the incarnation of our Lord Jesus Christ.”
[…] Thus, we may reasonably doubt the utility of this question, and this gets us to another approach that marginalizes the doctrine of the church (ecclesiology) and sacraments: the Pietists and their modern neo-evangelical children. The original Reformation evangelicals, e.g., Luther, Bucer, Calvin, Melanchthon, et al., all had a high view of the visible church and the sacraments. They thought the sacraments were worth arguing about and that is why, in the Belgic Confession, the Reformed churches speak as they do. They did not regard the visible church as a mere appendage to one’s personal experience or small group. They saw the visible church as that embassy of the Kingdom of God instituted by Christ, through which, by divine ordination, the elect are brought to new life and true faith.