Cyril of Alexandria

Now His disciples supposed that He would go to Jerusalem, and would at once make a manifestation of the kingdom of God. To rid them therefore of this belief, He informs them that it became Him first to suffer the Life-giving Passion, then to ascend to the Father and shine forth from above, that He might judge the world in righteousness. Hence He adds, But first must he suffer many things, and be rejected of this generation.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

Then, since He had foretold terrible calamities for them, comforting them and urging them to endure these manfully, He sets Himself before them as an example. Do not marvel, He says, if such difficulties befall you as to make you wish for the return of My present sojourn with you. For I Myself, who am to appear as lightning, must first suffer many things and be rejected, and then come in that glory. …

Commentary on Luke ↗

Bede

But first, it is necessary for him to suffer many things and be rejected by this generation. The Lord, speaking much about the glory of His advent, also took care to insert a few words about the terror of His passion so that when they would see Him dying, whom they had heard would be glorified, the pain of the passion would be mitigated by the hope of the promised glory, and at the same time, they themselves might prepare, if they love the glory of the kingdom, not to dread the danger of death. …

On the Gospel of Luke ↗

Bonaventure

Second, as to the order of the appearing, he adds: But first he must suffer many things and be rejected by this generation. This is understood of Christ the head and the perfidious generation of the Jews, from whom he suffered many things, of whom Deuteronomy thirty-two says: "It is a perverse generation, and unfaithful children"; and Proverbs thirty: "A generation that curses its father, and that does not bless its mother." By this generation Christ suffered many reproaches, insults, blows, and torments, according to Isaiah …

Commentary on Luke, Chapter 17 ↗