They must therefore be forgiven in a middle state, Purgatory. Belief in the existence of Purgatory is a continuous and solemn teaching of the Church. From Saint Paul , the early Fathers, the Doctors of the Church, on through the ages, the Church has taught the existence of Purgatory, and the correlated doctrine of the usefulness of praying for the dead. From the beginning Christians prayed for the dead at the Holy Sacrifice of the Mass. The oldest books used at Mass contain prayers for the dead.
Reason demands belief in the existence of Purgatory.
God's mercy and justice forbid it. Therefore reason concludes the existence of a middle and temporary state of expiation, where the soul is cleansed from all stain of sin before it can be admitted into the perfect holiness and bliss of Heaven. Among nearly all peoples there has persisted a belief that souls must undergo some sort of purification after death. This would point to the doctrine of Purgatory. Not only is this what Catholics believe, but the Greek story of Prometheus implies a place of purgation.