Following Augustine, the next major figure in the development of the doctrine of purgatory is St. Thomas Aquinas. Aquinas’ teaching key roles in the development of the concept of purgatory in two ways. First, his view of justification (the concept of progressive justification based off his understanding of the Latin term iustificare and appropriation of Aristotelian physics to it), and second, Aquinas developed what may be the most complete and explicit doctrine of purgatory in the church prior to the council of Florence or the council of Trent. Aquinas understood justification as a cause and effect process, one which involved motion either towards sanctification and therefore towards God or away from God. For Aquinas, justification was not a once and for all piece of the salvific puzzle, but was instead much more of a journey. For those who only committed venial sins and regularly received the sacraments, the journey was shorter and easier (although still not devoid of purgatorial cleansing), while for those who perhaps committed mortal sins but received the second plank for shipwrecked souls through works of penance, the process was much longer and more arduous. Given that Aquinas believed justification involved being made righteous through a process rather than through the once for all covering of Christ, it is understandable that he would necessarily have to develop a doctrine of purgatory, lest nearly all men end up in eternal damnation because they failed to achieve moral perfection prior to their death.
Aquinas’ work is also key to understanding the doctrine of purgatory not only as the medieval precursor to Roman Catholic teaching prior to and following the reformation, but also because it is the single most detailed teaching on purgatory that came from anywhere outside of a council of the church. While other theologians in history at least had a concept of a temporary place of (painful) punishment and cleansing, they rarely offered any more detail than that. Aquinas has two significant articles in his Summa Theologica on the topic, and we can see that they are truly the ground of future developments of the doctrine of purgatory. Aquinas explains the necessity of purgatory:
For if the debt of punishment is not paid in full after the stain of sin has been washed away by contrition, nor again are venial sins always removed when mortal sins are remitted, and if justice demands that sin be set in order by due punishment, it follows that one who after contrition for his fault and after being absolved, dies before making due satisfaction, is punished after this life.
From Aquinas we gather a full understanding of Roman Catholic teaching on the necessity of purgatory. The council of Trent lacks substantial teaching in relation to the theology of purgatory, but instead defers to the doctrine “transmitted by the holy Fathers and sacred councils” as both the source and the ultimate authority on the subject. Whether this refers particularly towards the patristic fathers or not is not entirely clear, but based off what limited teaching on this doctrine we find in their works, it is fair to infer that they are referring primarily to the teachings of Thomas Aquinas and St. Augustine. The teaching of Aquinas, Augustine, and subsequently the dogma of the Roman Catholic Church is that the demands of justice require sinners to be truly made righteous before God before they are able to enter into paradise. Interestingly enough, the contrition of the sinner and the absolution that they receive is somehow not sufficient to justify the person and make them righteous. Why Aquinas and other Roman Catholic theologians do not consider this absolution to be a sufficient cause to make the sinner righteous is difficult to grasp. It is understood in Aquinas’ theology that the sacraments, as an infusion of grace are an efficient cause towards justification and ultimately eternal life, but the fact that (based off his doctrine of purgatory) the work of Christ and additional sacraments which have been added on are insufficient to immediately merit salvation is tough to reasonably understand.
Aquinas may not have been the first to speak of purgatory as something similar to hell with a time limit, but he was among the first to explicitly teach that the cleansing which happens in purgatory is through fire. Aquinas is even willing to allow for the possibility that purgatory and hell may be the same place, but simply used for different purposes (affliction versus cleansing) and occupied by people in different states (those who are bound eternally to hell, versus those who have received some measure of grace through baptism and the sacraments). What is surprising however is that subsequent Roman Catholic councils dealing with this doctrine did not defer to Aquinas’ traditional teaching in this area, but instead determined not to teach beyond the conciliar definitions of the church. We can clearly see that the teachings of Aquinas on purgatory were both highly influential on the Roman Catholic Church’s development of the doctrine, and simultaneously beyond the bounds of that which Rome was willing to teach on the subject as they made clear at Florence and Trent. Whether or not Rome accepted all the teachings of Aquinas in this area, it remains clear that (with the possible exception of Augustine, although it is close) St. Aquinas had the most substantial influence on the doctrine of purgatory of any theologian in the history of the church.