Image: Baroque fresco, 1749, Simon Benedikt Faistenberger. Saint Ulrich am Pillersee parish church: Virgin Mary gives a black penitence belt to Saint Augustine and his mother Saint Monica. At the bottom, with this belt Saint Nicholas of Tolentino is freeing poor souls from the purgatory.
One thing we see through the history of the Church is that, with certain doctrines, something changes in the way they are understood. Take, for example, purgatory.
In the Catechism of the Catholic Church, you might be surprised to know that there are only three paragraphs devoted to it (1030, 1031, and 1032). The first states that those who die in God’s friendship, but are still imperfectly purified, are assured of eternal salvation. They undergo a purification to achieve the necessary holiness.