Gregory the Great — as excerpted in the Catena Aurea on Matthew 24:23-28

“(Hom. in Ev. xxxv. i.) Or, because the heart of the elect is assailed with fearful thoughts, yet their faithfulness is not shaken, the Lord comprehends both under the same sentence, for to waver in thought is to err. He adds, If it were possible, because it is not possible that the elect should be taken in error”