Gregory the Great was a saint, church father, pope, liturgist, theologian, and patron of missionary work in pagan lands.
In the passage cited below, Saint Gregory discusses purgatory. First, he states that there is "cleansing fire" that "purges" away "minor faults". Second, he cites Scripture to substantiate the doctrine. Third, he makes a logical argument from the words of Christ demonstrating that sins can and will be forgiven in the world to come.
"Each one will be presented to the Judge exactly as he was when he departed this life. Yet, there must be a cleansing fire before judgment, because of some minor faults that may remain to be purged away.What's the take-home principle? It's this. The Church Fathers already had a robust theology of post-mortem purgation by the sixth century and they found it justified in Sacred Scripture.
Does not Christ, the Truth, say that if anyone blasphemes against the Holy Spirit he shall not be forgiven "either in this world or in the world to come" (Mt 12:32)? From this statement we learn that some sins can be forgiven in this world and some in the world to come. For, if forgiveness is refused for a particular sin, we conclude logically that it is granted for others. This must apply, as I said, to slight transgressions."
Pope Saint Gregory the Great, Dialogues, 4:39 (A.D. 594).
For more Scriptural proof for Purgatory visit: The Apostle Paul on Purgatory and Prayers for the Dead.