31 I am disposed, after careful examination, to doubt whether the expression so often used by the Lord, "the kingdom of heaven," can be found in these books. It is said, indeed, "Love wisdom, that you may reign for ever." And if eternal life had not been clearly made known in the Old Testament, the Lord would not have said, as He did even to the unbelieving Jews: "Search the Scriptures, for in them ye think that you have eternal life, and they are they that testify of me." And to the same effect are the words of the Psalmist: "I shall not die, but live, and declare the works of the Lord." And again: "Enlighten my eyes, lest I sleep the sleep of death." Again, we read, "The souls of the righteous are in the hand of the Lord, and pain shall not touch them;" and immediately following: "They are in peace; and if they have suffered torture from men, their hope is full of immortality; and after a few trouble, they shall enjoy many rewards." Again, in another place: "The righteous shall live for ever, and their reward is with the Lord, and their concern with the Highest; therefore shall they receive from the hand of the Lord a kingdom of glory and a crown of beauty." These and many similar declarations of eternal life, in more or less explicit terms, are found in these writings. Even the resurrection of the body is spoken of by the prophets. The Pharisees, accordingly, were fierce opponents of the Sadducees, who disbelieved the resurrection. This we learn not only from the canonical Acts of the Apostles, which the Manichæans reject, because it tells of the advent of the Paraclete promised by the Lord, but also from the Gospel, when the Sadducees question the Lord about the woman who married seven brothers, one dying after the other, whose wife she would be in the resurrection. As regards, then, eternal life and the resurrection of the dead, numerous testimonies are to be found in these Scriptures. But I do not find there the expression, "the kingdom of heaven." This expression belongs properly to the revelation of the New Testament, because in the resurrection our earthly bodies shall, by that change which Paul fully describes, become spiritual bodies, and so heavenly, that thus we may possess the kingdom of heaven. And this expression was reserved for Him whose advent as King to govern and Priest to sanctify His believing people, was ushered in by all the symbolism of the old covenant, in its genealogies, its typical acts and words, its sacrifices and ceremonies and feasts, and in all its prophetic utterances and events and figures. He came full of grace and truth, in His grace helping us to obey the precepts, and in His truth securing the accomplishment of the promises. He came not to destroy the law, but to fulfill it.
Source: Reply to Faustus the Manichaean (New Advent)