Theophylact of Ohrid

For My second coming, most radiant and most glorious, will not be confined to any one place, but just as lightning is not hidden but appears from one end of the earth to the other, so bright and manifest will My second coming be, and it will be hidden from no one. Therefore, do not yield to the temptations of false christs. Previously I appeared in a manger and for thirty years was in humiliation, but then it will not be so: I will come in all glory, accompanied by angelic hosts, and in a single instant.

Commentary on Luke ↗

Bede

And he well says, that lighteneth out of the one part under heaven, because the judgment will be given under the heaven, that is, in the midst of the air, as the Apostle says, We shall be caught up together with them in the clouds. (1 Thess. 4:17.) But if the Lord shall appear at the Judgment like lightning, then shall no one remain hidden in the deep of his heart, for the very brightness of the Judge pierces through him; we may also take this answer of our Lord to refer to His coming, whereby He comes daily into His Church. …

Catena Aurea by Aquinas ↗

Bonaventure

For as lightning etc. After the rebuke of Jewish curiosity, there is here added the reason for the rebuke, which is taken from the manner of coming of the Son of God himself, who will not be able to be hidden when he comes, nor foreseen beforehand. This part is divided into three: in the first of which he treats of the manner of appearing; in the second, of the manner of coming, at the words: And as it was in the days of Noah; in the third, of the manner of escaping, at the words: In that day etc. …

Commentary on Luke, Chapter 17 ↗