From what has been said, it will be manifest to intelligent hearers how we have to answer the following: “All the rest of the race will be completely burnt up, and they alone will remain.” It is not to be wondered at, indeed, if such thoughts have been entertained by those among us who are called in Scripture the “foolish things” of the world, and “base things,” and “things which are despised,” and “things which are not,” because “by the foolishness of preaching it pleased God to save them that believe in Him, after that, in the wisdom of God, the world by wisdom knew not God,” — because such individuals are unable to see distinctly the sense of each particular passage, or unwilling to devote the necessary leisure to the investigation of Scripture, notwithstanding the injunction of Jesus, “Search the Scriptures.” The following, moreover, are his ideas regarding the fire which is to be brought upon the world by God, and the punishments which are to befall sinners.
And perhaps, as it is appropriate to children that some things should be addressed to them in a manner befitting their infantile condition, to convert them, as being of very tender age, to a better course of life; so, to those whom the word terms “the foolish things of the world,” and “the base,” and “the despised,” the just and obvious meaning of the passages relating to punishments is suitable, inasmuch as they cannot receive any other mode of conversion than that which is by fear and the presentation of punishment, and thus be saved from the many evils (which would befall them). The Scripture accordingly declares that only those who are unscathed by the fire and the punishments are to remain—those, viz., whose opinions, and morals, and mind have been purified to the highest degree; while, on the other hand, those of a different nature— those, viz., who, according to their deserts, require the administration of punishment by fire— will be involved in these sufferings with a view to an end which it is suitable for God to bring upon those who have been created in His image, but who have lived in opposition to the will of that nature which is according to His image. And this is our answer to the statement, “All the rest of the race will be completely burnt up, but they alone are to remain.”
Source: Against Celsus (New Advent)