The Formal Pre-Vatican II conciliar teachings of the Catholic Church concerning purgatory:
"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.
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).
From the Second General Council of Lyon and the Profession of Faith by Michael Paleologus, Second Part:
But because of various errors, introduced by some through ignorance and by others out of malice, she says and preaches that those who after Baptism lapse into son must not be rebaptized, but obtain pardon for their sins through true penance; that if being truly repentant, they die in charity before having satisfied by worthy fruits of penance for their sins of commission and omission, their souls are cleansed after death by purgatorial and purifying penalties, as Brother John has explained to us; and that to alleviate such penalties the acts of intercession (suffragia) of the living faithful benefit them, namely the sacrifices of the Mass, prayers, alms and other works of piety which the faithful are wont to do for the other faithful according to the Church's institutions.
As for the souls of those who, after having received holy baptism, have incurred not stain of sin whatever, and those souls who, after having contracted the stain of sin, have been cleansed, either while remaining still in their bodies or after having been divested of them as stated above, they are received immediately (mox) into heaven."
The Council of Florence (1439) repeats this formulary.
The Council of Trent (1563) Decree on purgatory:
The Catholic Church, instructed by the Holy Spirit and in accordance with sacred Scripture and the ancient Tradition of the Fathers, has taught in he holy Councils and most recently in this ecumenical Council that there is a purgatory and that the souls detained there are helped by the acts of intercession (suffragia) of the faithful and especially by the acceptable sacrifice of the altar. Therefore this holy Council commands the bishops to strive diligently that the sound doctrine of purgatory, handed down by the Holy Fathers and the sacred Councils, be believed by the faithful and that it be adhered, taught and preached everywhere. But let the more difficult and subtle questions which do not make for edification and, for the most part, are not conducive to and increase of piety (cf. 1 Timothy 1.4), be excluded from the popular sermons to uneducated people. Likewise they should not permit opinions that are doubtful and tainted with error to be spread and exposed. As for those things that belong to the realm of curiosity or superstition, or smack of dishonorable gain, they should forbid them as scandalous and injurious to the faithful