The history of Christianity has favoured paedobaptism—that act of baptizing an infant who belongs to a Christian family. This doesn’t mean the earliest church always baptized their infants. During the fourth and fifth centuries, Christians would often wait to be baptized until later in life as fire insurance. Even Gregory of Nazianzus, known to the eastern church as “The Theologian” and framer of the language of the dual nature of Christ, was baptized late into his life. I would also suggest that the Didache a assumes credobaptist position—the act of baptizing someone on the confession of faith. In any case, the history of Christianity does not define dogma, though it definitely helps shapes it. The Bible alone is the final authority on such matters.
And I would argue that a prophecy of Jeremiah drives home the conclusion that the church should only baptize those who confess faith, which would not include infants.
So here’s my argument. Jeremiah 31 contains a prophecy of the New Covenant, the covenant that Jesus inaugurated in Luke 22:20, which probably was ratified at the cross when Jesus died (“in my blood”). The church, then, directly participates in the New Covenant: we are “ministers of the New Covenant” (2 Cor 3:6). If the church participates in the New Covenant, what does that covenant entail?
Jeremiah 31:34 provides one entailment of the New Covenant: “And no longer shall each one teach his neighbor and each his brother, saying, ‘Know the LORD,’ for they shall all know me, from the least of them to the greatest, declares the LORD. For I will forgive their iniquity, and I will remember their sin no more.”
The New Covenant differs from older covenants in this way (among other things): everyone knows the Lord in the New Covenant. Nobody in the New Covenant needs to know the Lord; they already do.
So here is the point. Only people who can know the Lord can be part of the New Covenant. It seems unlikely to me that infants of believing families know of the Lord and don’t need to learn of him. All believers, all people in the New Covenant know God.
If baptism is the sign that you are in the New Covenant, how can you baptize an infant who does not know the Lord? The answer is that you cannot. I wonder if this is why Luther proposed that infants can believe through the sacrament of baptism, because he knew that an infant had to believe to participate in the New Covenant?
I find it far simpler to simply admit that only those who are mature enough to confess Jesus as Lord can be part of the New Covenant, which excludes infants from baptism.
Attribution for the opening picture: This Wikipedia and Wikimedia Commons image is from the user Chris 73 and is freely available at //commons.wikimedia.org/wiki/File:Baptismal_Font_Magdeburg.jpg under the creative commons cc-by-sa 3.0 license.