Water baptism is for believers. A person must believe, then they are baptized.
How is an infant supposed to believe, when they can’t even talk?
Again, pouring water over the head of a baby or even immersing a full grown adult in water is useless unless they believe.
The Jews practised infant circumcision, as mandated to Abraham (Genesis 17:12), reaffirmed in the Mosaic Law (Leviticus 12:3), and demonstrated by the circumcision of Jesus on His eighth day (Luke 2:21). Without circumcision no male was allowed to participate in the cultural and religious life of Israel.
The rite of circumcision as the doorway into the Old Covenant was replaced in the New Covenant with the rite of Baptism — both applied to infants. St. Paul makes this correlation: “In him also you were circumcised with a circumcision made without hands, by putting off the body of flesh in the circumcision of Christ; and you were buried with him in baptism” (Colossians 2:11–12).
When Peter preached under the inspiration of the Holy Spirit on the Day of Pentecost he was speaking to a Jewish audience (Acts 2:5–35). Peter announced, “Repent, and let each of you be baptised in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit. For the promise is for you and your children” (Acts 2:38–39). The Jews would have been dismayed had the New Covenant not included their children, especially since it was promised to them, and the New Covenant was to be an improvement over the Old in which they were included.
The New Testament frequently implies that adults and children were included in the rite of Baptism. For example, when the head of a household converted and was baptised, his entire household was also baptised with him (Acts 16:15, 33; 1 Corinthians 1:16). The inference of course, especially based on Jewish understanding of the family and covenants, would include the aged, the adults, the servants, and the infants. If the practice of Infant Baptism had been illicit or prohibited it would surely have been explicitly forbidden, especially to restrain the Jews from applying Baptism to their infants as they did circumcision. But we find no such prohibition in the New Testament nor in the writings of the Fathers — a silence that is very profound.
Many commentators see an allusion to Infant Baptism in the words of St. Luke, “Now they were bringing even infants to him that he might touch them; and when the disciples saw it, they rebuked them. But Jesus called them to him, saying, ‘Let the children come to me, and do not hinder them; for to such belongs the kingdom of God’" (Luke 18:15–16). In the early Church this passage was understood as a command to bring the infants to Christ for Baptism. The very first time this passage shows up in Christian literature (circa 200 AD), it is used in reference to Infant Baptism (Tertullian, De Baptismo 18:5). Even though Tertullian espoused a later baptism for children, he acknowledged that Infant Baptism was already the universal practice and does not try to avoid the interpretation of this verse’s reference to Infant Baptism. The Apostolic Constitutions (circa 350 AD) taught that children should receive baptism based on the words of Jesus, “Do not hinder them” (VI 15.7)
In the middle of the second century Infant Baptism is mentioned not as an innovation, but as a rite instituted by the apostles. Nowhere do we find it prohibited and everywhere we find it practised. Early in the nascent Church we have St. Irenaeus (circa 130-200 AD) who provides a very early witness to Infant Baptism, based on John 3:5. Irenaeus wrote, “For He [Jesus] came to save all through means of Himself — all, I say, who through Him are born again to God, — infants, and children, and boys, and youths, and old men” (Against Heresies, 2, 22, 4).
Origen (circa 185-254 AD) who had travelled to the extents of the Roman Empire wrote with confidence, “The Church received from the Apostles the tradition [custom] of giving Baptism even to infants. For the Apostles, to whom were committed the secrets of divine mysteries, knew that there is in everyone the innate stains of sin, which must be washed away through water and the Spirit” (Commentary on Romans 5, 9).
St. Augustine confirmed the ubiquitous teaching of the Church when he wrote, “This [infant baptism] the Church always had, always held; this she received from the faith of our ancestors; this she perseveringly guards even to the end” (Augustine, Sermon. 11, De Verb Apost) and “Who is so impious as to wish to exclude infants from the kingdom of heaven by forbidding them to be baptised and born again in Christ?” (Augustine, On Original Sin 2, 20).
Throughout Christian history, only a very few have opposed Infant Baptism. The opposition resides mainly in those of Anabaptist heritage which originated in the sixteenth century and who were strongly opposed by Reformers Martin Luther and John Calvin who both taught and practised Infant Baptism. The Anabaptists’ opposition to the baptism of infants lies mainly in their belief — unsupported by Scripture and with no supporting evidence from the practice of the early Church — that one has to be of sufficient age to exercise personal faith in Christ and make a personal confession at baptism. Nowhere is this taught in Scripture that only adults can receive baptism. To hold this extreme view is to be outside the continuity of historical Christianity.