The First Baptist Theologian: Tertullian of Carthage (c.160 - c.225)
A Baptist preacher I know once informed me that while Presbyterians trace their lineage to the 16th century reformer John Calvin, Baptists trace theirs to the first century prophet John the Baptist. Though most Baptists I know aren't hugely invested in the antiquity or catholicity of their peculiar beliefs, the implications of this preacher's claim for the validity of our respective traditions -- and specifically our convictions regarding baptism -- were intended to be fairly obvious. I responded with a remark about the glaring absence of Baptist fossils in the church historical record between the 1st and 16th centuries. He thought discussion had turned to a subject he enjoyed even more than that of denominational differences -- namely, evolution -- and the conversation moved on.
If the first person to administer Christian baptism, at least to a convert if not to his/her children, counts as the first "Baptist," then that title does very likely belong to John or one of Christ's apostles (depending on how one relates John's baptism to Christian baptism; cf. Acts 19.1-7). If, however, by "Baptist" we mean one who by conviction claims that none but those who make a mature and credible profession of faith should receive the sacrament of baptism, then I posit that the very first proven "Baptist" theologian was the late 2nd/early 3rd century African theologian Tertullian.
Tertullian wrote a book on baptism (De baptismo) in the first decade of the 3rd century --roughly 170 years after Christ instituted the sacrament of baptism (Matt. 28.19). In that work we encounter the earliest argument to my knowledge against the practice of baptizing infants; which practice, his argument as such clearly indicates, was rather common in the church in Carthage (and presumably beyond), despite the rather extraordinary, recent claim by Michael Haykin that the "Ancient Church largely" knew only believer's baptism "up until the fifth century."
It may be better to delay Baptism; and especially so in the case of little children. Why, indeed, is it necessary -- if it be not a case of necessity -- that the sponsors too be thrust into danger, when they themselves may fail to fulfill their promises by reason of death, or when they may be disappointed by the growth of an evil disposition? [...] Let [children] come, then, [when] they grow up...; let them become Christians when they [are] able to know Christ! Why does the innocent hasten to the remission of sins?
Tertullian's suggestion that infants -- those brought to the baptismal font not by virtue of their own choice but by the hands of "sponsors" -- be denied baptism until later in life rests on a number of significant assumptions: 1) that baptism positively effects the remission of sins; 2) that infants, though born relatively "innocent" (Tertullian lacks a developed doctrine of original sin), will often grow up to commit fairly significant sins; 3) that since one cannot be re-baptized for "the remission of sins" later in life, one might -- depending on the gravity of his/her sins later in life -- cut themselves off from the hope of salvation by their post-baptismal sins.
Tertullian argues, then, that baptism would best be postponed until one reaches an age and station in life where he/she is less likely to make their baptism superfluous by sinning so seriously after baptism that he/she ends up damned. If parents are prevented (by death) from fulfilling their own promise to raise their child in the nurture and admonition of the Lord, or if, despite their best efforts at parenting, an "evil disposition" develops in their child, then let that child get the serious sins out of his/her system and only then, when better prepared to live the Christian life, be baptized. Tertullian at this stage in his career believed that God would forgive every baptized person one serious sin (adultery, apostasy, etc.) in their Christian life. He later came to the conviction that God would forgive no serious sins after baptism. Either way, best to commit your serious sins before baptism and let them be washed away by the same.
Of course, the force of Tertullian's logic could lead one to postpone baptism until rather late in life (though that would involve something of a gamble, since one might die unexpectedly, thereby losing the opportunity while living to have his/her sins remitted). In fact, Tertullian himself extends the logic of his argument to young, unmarried persons, who -- he believes (or perhaps remembers) -- might find themselves particularly prone to serious sexual sin:
For no less cause should the unmarried also be deferred [from baptism], in whom there is an aptness to temptation, ...until either they are married or are better strengthened for continence. Anyone who understands the seriousness of Baptism will fear its reception more than its deferral.
Whatever the (de)merits of Tertullian's theology of baptism, one thing is particularly noteworthy regarding his argument against infant baptism. That is the failure to advance what would have been, in his day, the most compelling point of all against the practice: that it was novel and/or unfamiliar to Christians in other parts of the Mediterranean world. In the ancient world, antiquity and uniformity were universally valued more than novelty and/or provinciality. Ancient Christians regularly signaled their preference for what was old and regular in their apologetic for Christian beliefs and practices against Judaism, Greco-Roman polytheism, and Gnosticism (think, for example, of the repeated claim among Christian apologists that Plato had learned his monotheism and doctrine of creation from reading their Scriptures). All Tertullian really needed to do to convince Carthaginian Christians to stop bringing their newborn children to the baptismal font was say, "Hey, we Christians never did this before!", or perhaps, "Hey, Christians aren't doing this in Alexandria or Rome!" (the beliefs and practices of which churches were well known to those in Carthage).
The reason Tertullian couldn't advance such an argument should be fairly obvious: Christians in each of these regions had been bringing their children to the baptismal font for as long as they could remember. The antiquity of the practice is reflected, I suggest, even in the slight hesitation Tertullian himself expresses about his argument: "It may be better," he writes, not "we must" or "we should." He himself would have baulked at urging too strongly the cessation of a well-practiced custom.
And so, while debates continue (as they must) about Scripture's teaching on the question of whether children of believers should receive the (new) covenant sign (as they had the old), we can at least make headway in determining who was the very first theologian to explicitly pose that children of believers should not be baptized. It was, I suggest, Tertullian (and after him, almost nobody until the sixteenth century...). Kudos to my Baptist friends for having such a distinguished, and old, theologian as the first certain defender of their cause, subsequent gaps in the Baptist fossil record notwithstanding.
And let me point out to my Baptist friends that the actual practice of paedobaptism is documented prior to Tertullian's rejection of paedobaptism. Indeed, it's documented by Tertullian's own work; Tertullian was, after all, objecting to something that was happening, not to a hypothetical scenario in which some harebrained believer took it into his or her head to baptize a baby. So the earliest documented practice of paedobaptism precedes the earliest explicit objection to paedobaptism. My tradition is older than yours.