One never becomes a member of the church by a resolution of the will or by birth—the latter is true only of certain state churches like Zurich, the prototype of the Volkskirche since the days of Zwingli, where today one can exercise all the rights of a church member except that which belongs to the clergy, without even being baptized. According to the testimony of the New Testament (1 Cor. 12:13), one becomes a member of the Church by Baptism. And the only theologically legitimate question, which also determines whether infant baptism is right or wrong, is, Who may be baptized: only those who can confess their faith in Jesus Christ, i.e., adults and older children who are able to do so, or also minor children, infantes in the strict sense?
So the question of infant baptism is a theological question, not merely one of practical sociology. Neither is it a question that is to be answered from history. Thomas Aquinas (Summa Theologica III Quaestio, 68:9) meets the objection that intention and faith are necessary for baptism with a quotation from the last chapter of the "Heavenly Hierarchy" of Dionysius the Areopagite, according to which the apostles approved the baptism of infants. But that is, to say the least, a tradition that cannot be verified.
However, Joachim Jeremias (b. 1900), (Hat die älteste Christenheit die Kindertaufe geübt? 1938) and W.F. Flemington (The New Testament Doctrine of Baptism, 1948) have advanced a mass of weighty arguments showing the probability that infant baptism, which is first mentioned expressis verbis by Irenaeus (c. ad 185), goes back to the apostolic age. There it was practiced following the pattern of the Jewish baptism of proselytes, which as is well known, was administered not only to adults but, in cases where entire families were admitted, to all the members of a household, including the children. The well-known examples of Lydia, the seller of purple, and of the jailer at Philippi (Acts 16) who were baptized together with all those in their households after they themselves had come to faith, come to mind here. [See Jeremias' Infant Baptism in the First Four Centuries and The Origins of Infant Baptism (SCM Press, 1960 and 1963)].
When Polycarp at the trial preceding his martyrdom testifies that he has been serving the Lord for eighty-six years (Mart. Pol., 9), the reference can only be to his membership in the church. Accordingly, his baptism must have taken place in the apostolic age, even prior to the year ad 70. The statement of Justin (Apol. 1:15) that at that time there were many Christians sixty and seventy years old who from the days of their childhood ematheteuthesan to Christo [who had become disciples of Christ] can refer only to members of the church who were baptized as children during the period between ad 80 and 90. We have already mentioned Irenaeus. He testifies that Christ came to save all, "all who by Him are regenerated unto God; babes (infantes), little children, boys, youths and men" (Adv. Haer., II 22:4). In the Church Order of his disciple Hippolytus (ca. 170- c. 235) the baptism of little children is mentioned in so many words. They are to be baptized before the adults, and their parents or some relative are to take their places at the "Amen" and confession of faith by speaking vicariously for them.
When Tertullian (ca. 155-220) in his Treatise on Baptism directs his polemics against the custom of infant baptism, he certainly is not attacking it as an innovation; even as, later on, Pelagius in his battle against Augustine's doctrine of original sin had to admit the argument that, after all, infants were baptized too; at least he does not deny the fact. Likewise, Origen (ca. 185- ca. 254) and Cyprian (ca. 200-258) presuppose the baptism of infants: the former in the claim later transmitted to the Middle Ages by Dionysius the Areopagite that the baptism of infants goes back to a tradition given by the Lord to his apostles (Commentary on Romans, 5:9); Cyprian in the well-known instruction given to Bishop Fidus (Ep. 64) not to defer baptism to the eighth day analogous to circumcision. Jeremias is right when he claims that a later introduction of infant baptism would have stirred up a great excitement and thus have left definite traces in the history of the Church. The results of church-historical investigation rather indicate that in the ancient church, precisely as in our modern mission fields, both forms of baptism, adult and infant, have always existed side by side. If that is true, then infant baptism must go back to the apostolic age. The baptism of children must then be included in the baptism of entire families, of which we have examples in the New Testament, even though the children are not specifically mentioned.
It is obvious from the above that the historical question whether the church of the apostolic age knew and practiced infant baptism must be answered in the affirmative with a very high degree of probability. But that fact in no wise decides the theological question concerning the right of infant baptism. After all, the church of Corinth in the days of the apostle Paul practiced a vicarious baptism for the dead [I Corinthians 15:29]. It is possible, therefore, that we are dealing here with a very ancient abuse. Theologically, infant baptism can be grounded only on Scriptural evidence which proves it to be a legitimate form of Baptism.