John Chrysostom

On this account then He adds another argument, which I have already mentioned, saying, "Where the man's treasure is, there is his heart also." For though none of these things should come to pass, saith He, thou wilt undergo no small harm, in being nailed to the things below, and in becoming a slave instead of a freeman, and casting thyself out of the heavenly things, and having no power to think on aught that is high, but all about money, usuries and loans, and gains, and ignoble traffickings. Than this what could be more wretched? …

Homily on the Gospel of Matthew 20 ↗

Augustine of Hippo

For if any does a work with the mind of gaining thereby an earthly good, how will his heart be pure while it is thus walking on earth? For any thing that is mingled with an inferior nature is polluted therewith, though that inferior be in its kind pure. Thus gold is alloyed when mixed with pure silver; and in like manner our mind is defiled by lust of earthly things, though earth is in its own kind pure.

Catena Aurea by Aquinas ↗

Jerome

For where your treasure is, there your heart will be also. This applies not only to money, but also to all passions and possessions. The stomach is the god of gluttons: there it has its heart, where its treasure is. The treasure of the luxurious is feasts. For the lascivious, it is playfulness; for the lover, it is desire. Everyone serves the one by whom he is conquered (2 Peter 2:19).

Commentary on Matthew ↗

Thomas Aquinas

For where thy treasure is. Here he wishes to show that we should treasure up in heaven and not on earth, first because of the harm that results, which is twofold. The first is the distraction of the heart; the second, alienation from God, at the words No man can serve two masters. Regarding the first, he does two things. First, he sets forth the harm of the distraction of the heart; secondly, he shows the magnitude of this harm, at the words The light of thy body. He says, therefore: I said that thieves break through, etc. …

Commentary on Matthew ↗