The Medieval and Modern (Dis)Connection: Purgatory

As a written and defined doctrine, purgatory first appeared between 1170 and 1180; however, documents hint at the idea far earlier in Christian history. As early as 731 A.D., in Bede’s Historia Ecclesiastica, the basic principles and characteristics of purgatory lay in place. At this stage in Catholicism purgatory was defined by three central components. The first principle understood that purgatory existed for the remission of lighter sins after death. Caesarius of Arles and Augustine the Great both wrote and defined such lighter sins, or slight sins, as sins without which man cannot live and to which “even the saints could not and would not always be immune.” These sins, in the early Church theologian minds, were understandable and therefore not meriting eternal damnation. Purgatory, thus, in its earliest essence, was a place for redemption of the lesser sins.
The second essential component of purgatory was the purgative and temporal nature of its punishment. To the medieval mind, the nature of purgatorial fire was extremely important for the laity as they were seen, in contrast to the papacy, as the ones who had to prepare for such punishments. Imperative to this understanding was its difference from the fires of Hell. Whereas Hell tortured sin-ridden souls as punishment for their deeds on Earth, purgatory fires purified repentant souls before admittance into Heaven.
Fire, as it should be understood in the Middle Ages, was a major device of purification and held great importance in the medieval concept of purgatory. In medicine, fire was used to cauterize wounds and for primitive attempts at sterilization. Quickly this idea of purification through fire leaked into Christianity. Just as Church leaders began burning heretics in order to purify their blasphemous notions, the fires of purgatory were thought of in the same light.
This second component quickly gave way to the sacramental linkage between penance and purgatory. Purgatory began to been seen in the 12th and 13th century as the place where penance was finished. It was very commonly believed that when a person died they still had penance for their sins to complete before being completely remitted of them. Purgatory, then, became the place for such remission and the completion of penance.
The third and final principal notion of purgatory in the Middle Ages was the availability of intercessory prayer for the help of souls in purgatory:

Instances of successful intercession for the dead are often recorded
for posterity by patristic writers such as Gregory or Bede, and because
of their undiminished popularity, these tales were often quotes quite
nonchalantly in Middle English didactic poetry.