The Angelic Doctor Against the Errors of the Greeks - Part Two - Chapter 40

That there exists a purgatory wherein souls are cleansed from sins not cleansed in the present life.

The power of this sacrament, however, is lessened by those who deny here exists a purgatory after death; for on the souls in purgatory special healing is conferred by this sacrament. For Gregory of Nyssa in his sermon on the dead says: “If anyone her in his frail life has been less than able to cleanse himself of sin, after departing hence, through the blazing fire of purgatory the penalty is the more quickly paid, the more and more the ever-faithful Bride offers to her Spouse in memory of his passion gifts and holocausts on behalf of the children she has brought forth for that Spouse by word and sacrament; just as we preach in fidelity to this dogmatic truth, so we believe.” 

Likewise Theodoret, Bishop of Cyr, commenting on that passage of 1 Cor. 3: 11: If any man’s work burn, etc., says thus: “The Apostle states that one is saved thus as through a blazing fire cleansing whatever accumulated through carelessness in life’s activity, or at least from the dust of the feet of earthly living. In this fire one remains so long as any earthly and bodily affections are being purged. For such a person holy Mother Church pays and devoutly offers peace offerings, and so through this such a one emerging clean and pure assists immaculate before the most pure eyes of the Lord of hosts.”