Sermon on the Mount: Treasure in Heaven

Memorial of Saint Aloysius Gonzaga, Religious - Matthew 6:19-23

After saying, 'Lay not up for yourselves treasure on earth', Jesus adds, 'where rust and moth destroy,' in order to show the insecurity of that treasure that is here, and the advantage of that which is in Heaven, both from the place, and from those things which harm. As though He had said; Why fear you that your wealth should be consumed, if you should give alms? You rather give alms, and they shall receive increase, for those treasures that are in Heaven shall be added to them, which treasures perish if you do not give alms. He said not, You leave them to others, for that is pleasant to men.

By the eye here we may understand our purpose; if that be pure and right, all our works which we work
according to are good. These He here calls the body, as the Apostle Paul speaks of certain works as members;
"Mortify your members, fornication and uncleanness" (Colossians 3:5). We should look then, not to what a person does, but with what mind he does it. For this is the light within us, because by this we see that we do with good intention what we do. For all which does make manifest is light. But the deeds themselves, which go forth to men's society, have a result to us uncertain, and therefore He calls them darkness; as when I give money to one in need, I know not what He will do with it. If then the purpose of your heart, which you can know, is defiled with the lust of temporal things, much more is the act itself, of which the issue is uncertain, defiled. For even though one should reap good of what you do with a purpose not good; it will be imputed to you as you did it, not as it resulted to him. If however our works are done with a single purpose, that is with the aim of charity, then are they pure and pleasing in God's sight.

Reference:
Homily 20 on Matthew (St. John Chrysostom)
On the Sermon on the Mount, Bk. 2, Ch. 16 (St. Augustine)