1 Faustus said: You quote from the Gospel the words, "Many shall come from the east and the west, and shall sit down with Abraham, and Isaac, and Jacob, in the kingdom of heaven," and ask why we do not acknowledge the patriarchs. Now, we should be the last to grudge to any human being that God should have compassion on him, and bring him out of perdition to salvation. At the same time, we should acknowledge in such a case the clemency shown in this act of compassion, and not the merit of the person whose life is undeniably blameworthy.
Thus, in the case of the Jewish fathers, Abraham, and Isaac, and Jacob, who are mentioned by Christ in this verse, supposing it to be genuine, although they led wicked lives, as we may learn from their descendant Moses, or whoever was the author of the history called Genesis, which describes their conduct as having been most shocking and detestable; we are ready to allow that they may, after all, be in the kingdom of heaven, in the place which they neither believed in, nor hoped for, as is plain enough from their books.
But then it must be kept in mind that, as you yourselves confess, if they did attain to what is spoken of in this verse, it was something very different from the nether dungeons of woe to which their own deserts consigned them, and that their deliverance was the work of our Lord Christ, and the result of His mystic passion. Who would grudge to the thief on the cross that deliverance was granted to him by the same Lord, and that Christ said that on that very day he should be with Him in the paradise of His Father? Who is so hard-hearted as to disapprove of this act of benevolence?
Still, it does not follow that, because Jesus pardoned a thief, we must approve of the habits and practices of thieves; any more than of the publicans and harlots, whose faults Jesus pardoned, declaring that they would go into the kingdom of heaven before those who behaved proudly. For, when He acquitted the woman accused by the Jews as sinful, and as having been caught in adultery, He told her to sin no more. If, then, He has done something of the same kind in the case of Abraham, and Isaac, and Jacob, all the praise is His; for such actions towards souls are becoming in Him who makes His sun to rise upon the evil and upon the good, and sends rain on the just and on the unjust. One thing perplexes me in your doctrine: why you limit your statements to the fathers of the Jews, and are not of opinion that the Gentile patriarchs had also a share in this grace of our Redeemer; especially as the Christian Church consists of their children more than of the seed of Abraham, Isaac, and Jacob.
You will say that the Gentiles worshipped idols, and the Jews the Almighty God, and that therefore Jesus had regard only to the Jews. It would seem from this that the worship of the Almighty God is the sure way to hell, and that the Son must come to the aid of the worshipper of the Father. That is as you please. For my part, I am ready to join you in the belief that the fathers reached heaven, not by any merit of their own, but by that divine mercy which is stronger than sin.
Source: Reply to Faustus the Manichaean (New Advent)