Theophylact of Ohrid

Saying that "the whole multitude of disciples" praised God, Luke calls disciples all the followers of Jesus in general, not only the twelve and not the seventy, but all the people who, either being in need of miracles or at times being carried along by His teaching, followed Jesus. Among them naturally there were also children, as the other Evangelists relate (Matt. 21:15).

Commentary on Luke ↗

Methodius of Olympus

But while these things were doing, and the disciples were rejoicing and praising God with a loud voice for all the mighty works that they had seen, saying, Blessed be the King that cometh in the name of the Lord; peace in heaven, and glory in the highest; the city began to inquire, saying, Who is this? stirring up its hardened and inveterate envy against the glory of the Lord. But when thou hearest me say the city, understand the ancient and disorderly multitude of the synagogue. They ungratefully and malignantly ask, Who is this? …

Methodius Oration on the Psalms ↗

Ambrose of Milan

For it pleased not the Lord of the world to be borne upon the ass's back, save that in a hidden mystery by a more inward sitting, the mystical Ruler might take His seat in the secret depths of men's souls, guiding the footsteps of the mind, bridling the wantonness of the heart. His word is a rein, His word is a goad. The multitude then acknowledging God, proclaims Him King, repeats the prophecy, and declares that the expected Son of David according to the flesh had come, saying, Blessed be the King that cometh in the name of the Lord.

Catena Aurea by Aquinas ↗

Bede

And as he was now approaching the descent of the Mount of Olives, the whole multitude of disciples began to rejoice and praise God with a loud voice. As the Lord descends from the Mount of Olives, the rejoicing and praising multitudes also descend, because with the author of mercy humbling himself by his own will, it is necessary for those who are most in need of mercy to imitate, as far as they can, the footsteps of His humility. …

On the Gospel of Luke ↗

Bonaventure

Second, as regards the expression of devotion in speech, there is added: And when he was now approaching the descent of the Mount of Olives, on account of the fittingness of the place, because it was near Jerusalem. Whence also he ascended from that place: Acts 1: "They returned to Jerusalem from the Mount of Olives, which is near Jerusalem, a Sabbath day's journey." By that descent, however, is understood the descent of the condescension of God's mercy, on account of which he ought rightly to be praised by us, according to that …

Commentary on Luke, Chapter 19 ↗