Short answer. Purgatory came late. The practice (praying for the dead so the not-yet-perfect might be cleansed) is ancient, but Purgatory as a place, a third realm between Heaven and Hell, crystallized only in the 12th-13th centuries. Historian Jacques Le Goff showed the very noun purgatorium surfaces around 1170 (The Birth of Purgatory, 1984); dogma followed at Florence (1439) and Trent (1563).
The practice is ancient; the place is medieval
The single most important distinction here is between a practice and a place. Jews and early Christians prayed for the dead long before anyone imagined a discrete third realm. The locus classicus is 2 Maccabees 12:39-45, where Judas Maccabeus collects money for a sin offering on behalf of fallen soldiers, a text the author frames as showing it is "a holy and pious thought" to pray for the dead so they might be released from sin (2 Macc 12:45). Catholic doctrine later cited this as scriptural warrant; Protestants noted, correctly, that 2 Maccabees sits outside the Hebrew canon. Either way, the passage proves a practice, not a cartography. As Le Goff insists, for over a millennium Christians prayed for the dead without any spatial notion of where those dead were being purified: "Purgatory" as a named place simply did not yet exist (The Birth of Purgatory, 1984). The genius of the medieval system was to take an ancient devotional reflex and give it real estate.
Augustine and Gregory: the seed of purgation
The theological seed (that some sins might be burned away after death) was planted in late antiquity, well before the place was mapped. Augustine, hesitating, allowed that there might be a purifying fire for certain lesser sins, though he wrote with conspicuous caution and never systematized it (City of God, Book 21; Enchiridion). It was Gregory the Great, around 600, who gave the idea narrative flesh: his Dialogues (Book IV) tell stories of souls appearing to the living, suffering temporary cleansing, freed by Masses said on their behalf. Le Goff treats Gregory as a pivotal figure precisely because he supplies the imagery (purgation as an experience with duration and relief) without yet supplying a fixed location (The Birth of Purgatory, 1984). This is the honest shape of the development: a devotional practice (prayer for the dead), then a theological intuition (a cleansing fire), then (centuries later) a place to house both. Each layer is real; the place is the latest and the most invented.
The noun purgatorium appears around 1170
Le Goff's most arresting evidence is grammatical. He tracked the Latin word and found that purgatorium as a noun (a thing, a place) does not appear in the surviving record until the late twelfth century, roughly 1170-1180; before that, Christians spoke only of purgatorial fire (ignis purgatorius), an adjective describing a process, not a destination (The Birth of Purgatory, 1984). The shift from adjective to noun is the birth of the place. He ties this to the intellectual ferment of the Paris schools, the rise of scholasticism, and (provocatively) to new urban habits of accounting and arithmetic that made it natural to imagine the afterlife as a measurable account to be paid down. Whether one accepts the full sociological reading, the philological core is hard to dispute: the word for the place is young. A doctrine whose central noun postdates the year 1000 cannot claim to be the unbroken teaching of the apostolic church, and Eastern Orthodoxy, which prays for the dead but never adopted purgatorium, is the living proof that the practice and the Latin place are separable.
Hardened into dogma: Florence and Trent
What scholasticism named, the councils fixed. The Second Council of Lyon (1274) and the Council of Florence (1439) (both negotiating reunion with the Greek East) produced the first conciliar definitions, affirming a state of purgation and the efficacy of prayers and Masses for the dead while pointedly leaving the nature of the fire undefined. The decisive moment came at the Council of Trent (1563), the Catholic Reformation's answer to Luther, who had attacked indulgences precisely because they presupposed Purgatory and the treasury of merit. Trent's decree affirmed that Purgatory exists and that souls there are helped by the suffrages of the faithful, but it deliberately forbade preaching the lurid, speculative details that had grown up around it. The arc, then, runs from a Jewish prayer for dead soldiers, through Augustine's hesitation and Gregory's ghost-stories, to a twelfth-century noun, to a sixteenth-century dogma. That is development, not deposit.
Where this sits against the older map
Purgatory is downstream of a much older quarrel about what death actually does to a person. If the soul is naturally immortal (the Greek inheritance Oscar Cullmann famously contrasted with the Hebrew hope of resurrection (Immortality of the Soul or Resurrection of the Dead?, 1958)) then a disembodied soul needs a holding-and-cleansing state between death and the end, and Purgatory fills the gap. If the future is bodily resurrection and final renewal, the architecture looks different. Here the resonance with an older Eastern hope is worth naming honestly (Zoroastrian eschatology imagined a universal final purgation, a river of molten metal at Frashokereti through which all pass and all are ultimately cleansed (Bundahishn 30)) but that is resonance you can lean on, not influence you can bank: Le Goff's documented sources are Augustine, Gregory, and the Paris schools, not the Magi. The deeper question Purgatory raises is whether post-mortem purification points toward the universal restoration the early universalists hoped for, or toward an eternal divided fate.
Common questions
Is Purgatory in the Bible?
The practice it rests on is ancient, but Purgatory as a place is not in the Bible. Catholic doctrine cited 2 Maccabees 12:39-45, where Judas Maccabeus collects money for a sin offering on behalf of fallen soldiers, framed as showing it is "a holy and pious thought" to pray for the dead. Protestants noted, correctly, that 2 Maccabees sits outside the Hebrew canon. Either way, the passage proves a practice, not a cartography.
When was Purgatory invented?
The distinction that matters is between a practice and a place. Praying for the dead is ancient, but Purgatory as a discrete third realm crystallized only in the 12th-13th centuries. Jacques Le Goff showed the very noun purgatorium, a thing and a place, does not appear in the surviving record until roughly 1170-1180; before that, Christians spoke only of purgatorial fire (ignis purgatorius), an adjective describing a process. The shift from adjective to noun is the birth of the place.
Did Augustine teach Purgatory?
He planted a seed, with conspicuous caution. Augustine allowed that there might be a purifying fire for certain lesser sins, but he wrote hesitantly and never systematized it (City of God, Book 21; Enchiridion). It was Gregory the Great, around 600, who gave the idea narrative flesh in his Dialogues (Book IV), supplying the imagery of purgation with duration and relief, but still without a fixed location.
When did Purgatory become official Catholic dogma?
The first conciliar definitions came at the Second Council of Lyon (1274) and the Council of Florence (1439), which affirmed a state of purgation while leaving the nature of the fire undefined. The decisive moment was the Council of Trent (1563), which affirmed that Purgatory exists and that souls there are helped by the suffrages of the faithful, but deliberately forbade preaching the lurid, speculative details. The arc is development, not deposit.
This page settles the history, not the theology: it shows that the practice of praying for the dead is ancient while the place called Purgatory and the very word for it are medieval, it does not, by itself, decide whether the underlying intuition of post-mortem cleansing is true.
Sources
2 Maccabees 12:39-45; Bundahishn 30; Augustine, City of God, Book 21 and Enchiridion; Gregory the Great, Dialogues, Book IV (c. 600); Jacques Le Goff, The Birth of Purgatory (1984); Oscar Cullmann, Immortality of the Soul or Resurrection of the Dead? (1958). Councils of Lyon (1274), Florence (1439), and Trent (1563).