St. John did not absolutely forbid that prayer should be made for those who “sin unto death,” since he knew that Moses, Jeremiah, and Stephen had so prayed, and he himself implies that forgiveness is not to be denied them.
44. Such intercessors, then, must be sought for after very grievous sins, for if any ordinary persons pray they are not heard.
45. So that point of yours will have no weight, which you take from the Epistle of John, where he says: “He who knows that his brother sins a sin not unto death, let him ask, and God will give him life, because he sinned not unto death. There is a sin unto death: not concerning it do I say, let him ask.” He was not speaking to Moses and Jeremiah, but to the people, who must seek another intercessor for their sins; the people, for whom it is sufficient they entreat God for their lighter faults, and consider that pardon for weightier sins must be reserved for the prayers of the just. For how could John say that graver sins should not be prayed for, when he had read that Moses prayed and obtained his request, where there had been wilful casting off of faith, and knew that Jeremiah also had entreated?
46. How could John say that we should not pray for the sin unto death, who himself in the Apocalypse wrote the message to the angel of the Church of Pergamos? “You have there those that hold the doctrine of Balaam, who taught Balac to put a stumbling-block before the children of Israel, to eat things sacrificed unto idols, and to commit fornication. So have you also them that hold the doctrines of the Nicolaitans. Repent likewise, or else I will come to you quickly.” Do you see that the same God Who requires repentance promises forgiveness? And then He says: “He that has ears let him hear what the Spirit says to the churches: To him that overcomes will I give to eat of the hidden manna.”
47. Did not John himself know that Stephen prayed for his persecutors, who had not been able even to listen to the Name of Christ, when he said of those very men by whom he was being stoned: “Lord, lay not this sin to their charge”? And we see the result of this prayer in the case of the Apostle, for Paul, who kept the garments of those who were stoning Stephen, not long after became an apostle by the grace of God, having before been a persecutor.
Source: On Repentance (New Advent)