Anyone familiar with Augustine’s writings against Pelagius will recognize when his methodology reappears in later writers. One of his more memorable proofs for original sin was infant baptism. The argument went something like this, “If they don’t have any sin, then why are we baptizing them for the remission of sins?” You can’t wash off something that isn’t there.
This was a simple but effective maneuver. Peter Brown argues in his biography on Augustine that it was the practice of infant baptism that secured popular support for Augustine. Just like with Athanasius, liturgical practice helped secure dogma, at least in the larger public arena.
Again, the argument works because the belief that baptism washes away sins is presumed. Baptismal Regeneration was a doctrine universally agreed on prior to similar agreement on the doctrine of original sin. Therefore it could serve as the ground of the argument for original sin.
I find it informative that Augustine could use the argument in the 4th century, but it is even more interesting to me that folks still used it in the 17th century. Edward Polhill did just that in England in 1678 (*Hint- This is about thirty years after the Westminster Assembly). Here’s his argument:
Our Saviour Christ instituted baptism, and that for infants; but if there be no original pollution in them, what need a washing ordinance for them? The washing of their bodies, whose pure, innocent, undefiled souls are incapable of spiritual washing, is but a shadow without substance, a sacrament without internal grace, a thing too insignificant for Christ the wisdom of God to institute. Hence, when the Pelagians on the one hand granted the baptism of infants, and on the other denied original sin, St. Austin saith, that hey spoke wonderful things. In sacramento salvatoris baptizantur, sed non salvantur, redimuntur sed non liberantur, lavantur sed non abluuntur; In our Saviour’s sacrament infants are baptized, but not saved; redeemed, but not delivered; washed but not cleansed. And a little after he asks, If they are saved, what was their sickness? If delivered, what their servitude? If cleansed, what their pollution? Take away the doctrine of original sin, and the baptism of infants seems to be a very ridiculous thing. To avoid this absurdity, the Pelagians asserted, That the baptism of infants was necessary, not because there was any original sin in them, but that they might be capable of the kingdom of heaven. But I answer, Where there is no defect, there is all due perfection. If infants are pure and free from all sin, then have they all their righteousness and rectitude which ought to be in them; and if they have so, they are, without baptism, capable of heaven; or if they were not, the baptismal washing, which imports pollution, seems to be a ceremony very unfit and incongruous to be applied to them who are without spot, or to render them apt for heaven.
~ A View of Some Divine Truths pg 58-59
So in a dramatic sort of irony, we could say that those who deny baptismal regeneration are the semi-Pelagians.