St. Thomas Aquinas wrote that the souls in purgatory can expect a twofold pain: the pain of loss, which is the delay of the Beatific Vision, and the pain of sense, which is the fire of purgatory. Aquinas observed in his Summa Theologica: “With regard to both the least pain of Purgatory surpasses the greatest pain of this life. For the more a thing is desired, the more painful is its absence” (Appendix 1, 2). St. Thomas also taught that the fire of purgatory is the same as the fire of hell. This is, unquestionably, quite frightening, but at least St. Thomas assured us the souls in purgatory will not be tormented by demons, as that punishment is reserved for the damned. Those in purgatory have resisted demonic temptation, so do not deserve to be plagued after death by forces over which they have triumphed in life.
Saints who have written on purgatory are nearly countless. They include John Chrysostom, Augustine, Nicholas of Tolentino (the patron of souls in purgatory) and, in more present times, St. Faustina. Those wishing to consider concise reflections on purgatory from these and other holy souls may be interested in a book by Father F.X. Schouppe, S.J., titled “Purgatory Explained.”