34 But then the Lord says, “Be not solicitous for your life what you shall eat, nor for the body, what you shall put on.” Rightly: because He had said above, “You cannot serve God and mammon.” For he who preaches the Gospel with an eye to this, that he may have whereof he may eat and whereof be clothed, accounts that he at the same time both serves God, because he preaches the Gospel; and mammon, because he preaches with an eye to these necessaries: which thing the Lord says to be impossible.
And hereby he who does for the sake of these things preach the Gospel is convicted that he serves not God but mammon; however God may use him, he knows not how, to other men's advancement. For to this sentence does He subjoin, saying “Therefore I say unto you, Be not solicitous for your life what you shall eat, nor for your body what you shall put on:” not that they should not procure these things, as much as is enough for necessity, by what means they honestly may; but that they should not look to these things, and for the sake of these do whatever in preaching of the Gospel they are bidden to do.
The intention, namely, for which a thing is done, He calls the eye: of which a little above He was speaking with purpose to come down to this, and saying, “The light of your body is your eye: if your eye be single, your whole body shall be full of light; but if your eye be evil, your whole body shall be full of darkness;” that is, such will be your deeds as shall be your intention for which you do them. For indeed that He might come to this, He had before given precept concerning alms, saying, “Lay not up for yourselves treasures on earth where rust and moth does corrupt, and where thieves break through and steal.
But lay up for yourselves treasure in heaven, where neither moth nor rust does corrupt, and where thieves do not break through nor steal. For where your treasure shall be, there will your heart be also.” Thereupon He subjoined, “The light of your body is your eye:” that they, to wit, which do alms, do them not with that intention that they should either wish to please men, or seek to have repayment on earth of the alms they do. Whence the Apostle, giving charge to Timothy for warning of rich men, “Let them,” says he “readily give, communicate, treasure up for themselves a good foundation for the time to come, that they may lay hold on the true life.” Since then the Lord has to the future life directed the eye of them which do alms, and to an heavenly reward, in order that the deeds themselves may be full of light when the eye shall be simple, (for of that last retribution is meant that which He says in another place, “He that receives you receives Me, and he that receives Me receives Him that sent Me.
He that receives a prophet in the name of a prophet shall receive a prophet's reward; and he that receives a righteous man in the name of a righteous man shall receive a righteous man's reward. And whosoever shall give to drink unto one of these little ones a cup of cold water only in the name of a disciple, verily I say unto you, his reward shall not be lost,”) lest haply after he had reproved the eye of them which bestow things needful upon the indigent both prophets and just men and disciples of the Lord, the eye of the persons to whom these things were done should become depraved, so that for the sake of receiving these things they should wish to serve Christ as His soldiers: “No man,” says He, “can serve two masters.”
And a little after: “You cannot,” says He, “serve God and mammon.” And straightway He has added, “Therefore I say unto you, be not solicitous for your life what you shall eat, nor for the body what you shall put on.”
Source: On the Work of Monks (New Advent)