Cyril of Alexandria

For to the Jews who held the first place have the Gentiles been preferred.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

These words are directed at us when we act in a manner inconsistent with the faith. For we too boast in the Law of Christ, but then trample upon it when we transgress it, and therefore, although we seem to be "first," as having received Christ and His teaching from our very swaddling clothes, we shall be "last," because the pagans who believed at the end of their lives and lived in a God-pleasing manner for the remaining time will be placed above us.

Commentary on Luke ↗

Bede

And behold, there are the last who will be first, and there are the first who will be last. The main sense of this sentence indeed is clear from the preceding statements: because the peoples of the Gentiles, living without faith for a long time, would be called to faith, and the Jews, holding the head of faith and justice through many generations, would be turned to the tail. …

On the Gospel of Luke ↗

Bonaventure

And because this will be to the confusion of the reprobate and the glory of the elect, therefore he adds: And behold, the last shall be first, and the first shall be last. The first are last on account of pride, and the last are first on account of humility: because it is said below in chapter 18: "Everyone who humbles himself shall be exalted, and he who exalts himself shall be humbled." Why is this? …

Commentary on Luke, Chapter 13 ↗