The passage quoted from St. John's Epistle is confirmed by another in which salvation is promised to those who believe in Christ, which refutes the Novatians who try to induce the lapsed to believe, although denying them pardon. Furthermore, many who had lapsed have received the grace of martyrdom, whilst the example of the good Samaritan shows that we must not abandon those in whom even the faintest amount of faith is still alive.
48. Since, then, we have spoken of the general Epistle of St. John, let us enquire whether the writings of John in the Gospel agree with your interpretation. For he writes that the Lord said: “God so loved this world, that He gave His only-begotten Son, that every one that believes in Him should not perish but have everlasting life.” If, then, you wish to reclaim any one of the lapsed, do you exhort him to believe, or not to believe? Undoubtedly you exhort him to believe. But, according to the Lord's words, he who believes shall have everlasting life. How, then, will you forbid to pray for him, who has a claim to everlasting life? Since faith is of divine grace, as the Apostle teaches where he speaks of the differences of gifts, for “to another is given faith by the same Spirit.” And the disciples say to the Lord: “Increase our faith.” He then who has faith has life, and he who has life is certainly not shut out from pardon; “that every one,” it is said, “that believes in Him should not perish.” Since it is said, Every one, no one is shut out, no one is excepted, for He does not except him who has lapsed, if only afterwards he believes effectually.
49. We find that many have at length recovered themselves after a fall, and have suffered for the Name of God. Can we deny fellowship with the martyrs to these to whom the Lord Jesus has not denied it? Do we dare to say that life is not restored to those to whom Christ has given a crown? As, then, a crown is given to many after they have lapsed, so, too, if they believe, their faith is restored, which faith is the gift of God, as you read: “Because unto you it has been granted by God not only to believe in Him, but also to suffer in His behalf.” Is it possible that he who has the gift of God should not have His forgiveness?
50. Now it is not a single but a twofold grace that every one who believes should also suffer for the Lord Jesus. He, then, who believes receives his grace, but he receives a second, if his faith be crowned by suffering. For neither was Peter without grace before he suffered, but when he suffered he received a second gift. And many who have not had the grace to suffer for Christ have nevertheless had the grace of believing on Him.
51. Therefore it is said: “That every one that believes in Him should not perish.” Let no one, that is, of whatever condition, after whatever fall, fear that he will perish. For it may come to pass that the good Samaritan of the Gospel may find some one going down from Jerusalem to Jericho, that is, falling back from the martyr's conflict to the pleasures of this life and the comforts of the world; wounded by robbers, that is, by persecutors, and left half dead; that good Samaritan, Who is the Guardian of our souls (for the word Samaritan means Guardian), may, I say, not pass by him but tend and heal him.
52. Perchance He therefore passes him not by, because He sees in him some signs of life, so that there is hope that he may recover. Does it not seem to you that he who has fallen is half alive if faith sustains any breath of life? For he is dead who wholly casts God out of his heart. He, then, who does not wholly cast Him out, but under pressure of torments has denied Him for a time, is half dead. Or if he be dead, why do you bid him repent, seeing he cannot now be healed? If he be half dead, pour in oil and wine, not wine without oil, that may be the comfort and the smart. Place him upon your beast, give him over to the host, lay out two pence for his cure, be to him a neighbour. But you cannot be a neighbour unless you have compassion on him; for no one can be called a neighbour unless he have healed, not killed, another. But if you wish to be called a neighbour, Christ says to you: “Go and do likewise.”
Source: On Repentance (New Advent)