Matthew 6:21

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? For in truth such an one will be worse off than any slave, bringing upon himself a most grievous tyranny, and giving up the chiefest thing of all, even the nobleness and the liberty of man. For how much soever any one may discourse unto thee, thou wilt not be able to hear any of those things which concern thee, whilst thy mind is nailed down to money; but bound like a dog to a tomb, by the tyranny of riches, more grievously than by any chain, barking at all that come near thee, thou hast this one employment continually, to keep for others what thou hast laid up. Than this what can be more wretched? However, forasmuch as this was too high for the mind of His hearers, and neither was the mischief within easy view of the generality, nor the gain evident, but there was need of a spirit of more self-command to perceive either of these; first, He hath put it after those other topics, which are obvious, saying, "Where the man's treasure is, there is his heart also;" and next He makes it clear again, by withdrawing His discourse from the intellectual to the sensible.

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.

He now teaches the benefit of almsgiving. He who places his treasure on earth has nothing to look for in Heaven; for why should he look up to Heaven where he has nothing laid up for himself? Thus he doubly sins; first, because he gathers together things evil; secondly, because he has his heart in earth; and so on the contrary he does right in a twofold manner who lays up his treasure in Heaven.

This must be understood not of money only, but of all our possessions. The god of a glutton is his belly; of a lover his lust; and so every man serves that to which he is in bondage; and has his heart there where his treasure is.