Augustine of Hippo

Or; The lesser are therefore taken as first, because the lesser are to be made rich.

Catena Aurea by Aquinas ↗

Jerome

Is your eye evil because I am good? So the last shall be first, and the first last. For many are called, but few are chosen. This is the same meaning as the parable of the prodigal son in Luke, where the older son envies the younger and does not want to receive him when he returns and accuses the father of injustice. And so that we may know that this is the meaning we have said, the title and the end of this parable agree. So the last shall be first, and the first last. For many are called, but few are chosen.

Commentary on Matthew ↗

Gregory the Dialogist

But after these things, what follows is very terrible: "For many are called, but few are chosen," because many come to faith, yet few are led to the heavenly kingdom. For behold, how many of us have gathered for today's feast, we fill the walls of the Church, yet who knows how few there are who are numbered in that flock of God's elect? For behold, the voice of all proclaims Christ, but the life of all does not proclaim Him. Most follow God with their voices, but flee from Him by their conduct. …

Forty Gospel Homilies, Homily 19 ↗