Gregory the Great
Catena Aurea · Markusevangelium 13
And then if any man shall say to you, Lo, here is Christ; or, lo, he is there; believe him not: For false Christs and false prophets shall rise, and shall shew signs and wonders, to seduce, if it were possible, even the elect. But take ye heed: behold, I have foretold you all things. But in those days, after that tribulation, the sun shall be darkened, and the moon shall not give her light, And the stars of heaven shall fall, and the powers that are in heaven shall be shaken. And then shall they see the Son of man coming in the clouds with great power and glory. And then shall he send his angels, and shall gather together his elect from the four winds, from the uttermost part of the earth to the uttermost part of heaven.
(v. Greg Hom. in Ezech. lib. i. 9) Why however is it said with a doubt if it were possible, when the Lord knows beforehand what is to be? One of two things is implied; that if they are elect, it is not possible; and if it is possible, they are not elect. (non potest, ap. Cat.) This doubt therefore in our Lord’s discourse expresses the trembling in the mind of the elect. And He calls them elect, because He sees that they will persevere in faith and good works; for those who are chosen to remain firm are to be tempted to fall by the signs of the preachers of Antichrist.