All Christians agree that we won’t be sinning in heaven. Sin and final glorification are utterly incompatible. Therefore, between the sinfulness of this life and the glories of heaven, we must be made pure. Between death and glory there is a purification.
Thus, the Catechism of the Catholic Church states: "All who die in God’s grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven. The Church gives the name purgatory to this final purification of the elect, which is entirely different from the punishment of the damned" (CCC 1030–1).
The doctrine of purgatory, or the final purification, has been part of the true faith since before the time of Christ.
Jews, Catholics, and the Eastern Orthodox have always historically proclaimed the reality of the final purification. It was not until the Protestant Reformation in the sixteenth century that anyone denied this doctrine.
As the quotes in this tract from the early Church Fathers show, purgatory has been part of the Christian faith from the very beginning.
Need a bunch for your parish? Order in packs of 50 here.
Why Do We Pray for the Souls in Purgatory? 10/30/2012
What is Hell and How Do I Get There? 08/19/2011
True or False: Our Lord Descended Into Hell 03/19/2010
Bless Me Father, for I Have Sinned 08/10/2009
All Souls Day 10/22/2007