“Augustine developed the idea of limbo,” some people popularly say.
No, he did not, although his theological stance on original sin, divine grace and baptism – plus his admitted theological unease about what he feared was the eternal destiny of unbaptised dead infants – allowed later theologians to enlarge the notion of limbo and to cite Augustine in what they proposed.
Much of what Augustine wrote about eternal life after physical death was prompted by his controversy against Pelagius and the Pelagian heresy, Augustine was defending the theology of original sin, wherein contention arose with Pelagius and his followers by Augustine’s upholding the necessity of infant baptism as a means of removing original sin.
To do so, Augustine had to hold that an infant who died unbaptised was damned; in this Augustine was theologically very strict and very legalistic.
Augustine pressed the Pelagian to acknowledge original sin by reasoning that it must exist (derived from Adam’s first sin) if God had to damn children who died too young to have committed any personal sin.
Augustine was unwilling to countenance a third or alternate destiny for unbaptised infants. Augustine’s theology depended on the idea of God’s grace, upon which all humans are totally dependent for salvation; no human being – whether adult or infant – has any claim or right to God’s grace, which is not granted or earned according to a person’s merits.
When, however, on occasions when he was speaking specifically about the eternal fate of unbaptised infants (i.e., and not raising the topic to combat Pelagians regarding original sin and divine grace) Augustine apparently recoiled from the harsh implications that a deceased unbaptised infant would be punished in the same way as a consciously-unrepentant and hardened adult sinner. He thought that somehow they must face a lesser punishment.
He requested St Jerome to tell him his teaching on the subject, and also asked likewise of Bishop Optatus in his Letter 190. The best that Augustine could suggest was that God’s ways were not fully revealed to humanity, and that this problem defied human understanding.
It can be conjectured that Augustine might never have approached this problem at all had he not drawn it into his larger area of debate with Pelagianism.
Certainly, that he would later be labelled as providing the theological ammunition for the existence of limbo would have greatly surprised him. Possibly his heart had wanted to proceed in that direction, but his theology did not allow him to do so.