Cyril of Alexandria

The disciples failing to understand the force of Christ's words, readily resort to their ideas of what was usual, and suppose that Christ is once more indicating such commands as it was His wont to give. For as the feast was near at hand, they expected He was ordering the one who had the bag to buy something of what was necessary for it, or at least was very probably bidding him discharge that duty of which Christ was ever careful, namely, to give to the needy what He could, according to the resources at His disposal. …

Commentary on the Gospel of John, Book 9 ↗

Theophylact of Ohrid

So no one understood the words "do what you do quickly," but they thought that the Lord was commanding Judas to buy something for the feast or to give something to the poor. For He cared greatly for the poor, and although He urged the others to have neither bag nor money, He Himself allowed a money box to be carried with Him, showing thereby that even one who is without possessions and has been crucified to the world must have great concern for this class of people — the poor.

Commentary on John ↗

Augustine of Hippo

Our Lord then had bags, in which He kept the oblations of the faithful, to supply the wants of His own followers, or the poor. Here is the first institution of ecclesiastical property. Our Lord shews that His commandment not to think of the morrow, does not mean that the Saints should never save money; but that they should not neglect the service of God for it, or let the fear of want tempt them to injustice.

Catena Aurea by Aquinas ↗

Thomas Aquinas

Now the Evangelist tells us what the disciples, ignorant of the real reason Jesus was speaking, thought he meant: some of the disciples thought that, because Judas had the money box, Jesus was telling him... Here we should note that the Lord God of Heaven, who feeds all living things, had a purse, not to own the things of earth, but to save the offerings of believers and so provide for his own necessities and the needs of others. This purse was in the care of Judas. …

Commentary on John ↗

Bonaventure

For some thought, because Judas had the money bag, as steward, that Jesus had said to him: Buy those things which are needed for the feast day, or that he should give something to the poor, because he had the money bag especially on their account: in which it is signified that from the purses of the Church it is lawful for prelates to buy what is needed, that is, what is necessary, and to give the remainder to the poor; and to them especially the commandment was given concerning the feeding of the poor; whence Sirach twenty-nine: …

Commentary on John, Chapter 13 ↗