“In Tertullian’s Montanist essay On the Soul a step is taken which leads to the concept of Purgatory in its proper sense, though even here we are not dealing with an idea which is straightforwardly identical with the teaching of the mediaeval councils. Tertullian’s starting point is Jesus’ parabolic advice to reconcile oneself with one’s opponent on the way to court, since otherwise:
…you will be thrown into prison. I tell you solemnly, you will not get out till you have paid the last penny.
Interpreting this text in terms of human destiny in the world to come was made easier by the fact that phylake, the word for ‘prison,’ was also one of the current terms for Hades. For Tertullian, who had become a rigorist, the text meant that the time between death and resurrection is a time of imprisonment in which the soul has the opportunity to pay the ‘last penny’ and so to become free for the resurrection. A new theological rationale is being offered for Hades, and this rationale ‘makes the interim state into a necessary purgatory for everyone.’ Cyprian of Carthage, dying in 258, removed Tertullian’s thesis from its rigorist context and gave it a new look on the basis of the tasks of a pastor in a period of persecution. In this way, Cyprian succeeded in eliminating the pagan element. He managed to work out the authentically Christian form of an insight which, though derived from the Church’s Jewish roots, had earlier seemed equivalent to the Greco-Roman conceptions. Cyprian’s contribution set the Western Church on its way. He asserted a definitive salvation for those who have died in faith, and notably for the martyrs. He was similarly clear about the definitiveness of Hell. His actual pastoral problem concerned the well-intentioned but weak, average Christians who did not find the strength to accept martyrdom in times of persecution. They had carried out the demands of the State religion, and thus had publicly denied Christ. Nonetheless, they wished to remain Christians and asked for reconciliation with the Church. The saying found in Matthew 5:26 offered Cyprian an occasion for thinking through a possible continuation of penance in the afterlife. Against the protesting voices of the rigorists, this enabled him to readmit the weak to communion with the Church. Certainly they cannot, in their present condition, enter into definitive communion with Christ. Their denial, their half-heartedness, stands in the way. But they are capable of purification. The penitential way of purification exists not only in this world but in the world to come. With this interpretation, that there is purification in the future life, the root concept of the Western doctrine of Purgatory is already formulated clearly enough.”
-Joseph Ratzinger, Eschatology: Death and Eternal Life.