In "On the City of God against the Pagans" (De Civitate Dei Contra Paganos), Book 21, Ch. 24, Augustine wrote,
For some of the dead, indeed, the prayer of the Church or of pious individuals is heard; but it is for those who, having been regenerated in Christ, did not spend their life so wickedly that they can be judged unworthy of such compassion, nor so well that they can be considered to have no need of it. As also, after the resurrection, there will be some of the dead to whom, after they have endured the pains proper to the spirits of the dead, mercy shall be accorded, and acquittal from the punishment of the eternal fire. For were there not some whose sins, though not remitted in this life, shall be remitted in that which is to come, it could not be truly said (Matt. 12:32), “They shall not be forgiven, neither in this world, neither in that which is to come.”
Nam pro defunctis quibusdam vel ipsius Ecclesiae vel quorumdam piorum exauditur oratio sed pro his, quorum in Christo regeneratorum nec usque adeo vita in corpore male gesta est, ut tali misericordia iudicentur digni non esse, nec usque adeo bene, ut talem misericordiam reperiantur necessariam non habere; sicut etiam facta resurrectione mortuorum non deerunt, quibus post poenas, quas patiuntur spiritus mortuorum, impertiatur misericordia, ut in ignem non mittantur aeternum. Neque enim de quibusdam veraciter diceretur, quod non eis remittatur neque in hoc saeculo neque in futuro, nisi essent quibus, etsi non in isto, tamen remittitur in futuro.
Source: English | Latin