And some of them of understanding shall fall, to test them, and to purify them, and to make them white, until the time of the end: because it is yet for the time appointed.
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Aquinas Study Bible
Our writers would have it understood that the small help shall arise under the reign of the Antichrist, for the saints shall gather together to resist him, and afterwards a great number of the learned shall fall. And this shall take place in order that they may be refined as by fire in the furnace, and that they may be made white and may be chosen out, until the time before determined arrives ---- for the true victory shall be won at the coming of Christ. (St. Jerome)