2 Having thus, you see, spoken generally, that He might persuade the hearer to be an uncorrupt judge of His words, and to sentence according to the very nature of the things; when he has made sure of his assent, then, and not till then, He discovers Himself. Thus He presently adds, “You cannot serve God and mammon.” Let us shudder to think what we have brought Christ to say; with the name of God, to put that of gold. But if this be shocking, its taking place in our deeds, our preferring the tyranny of gold to the fear of God, is much more shocking.
“What then? Was not this possible among the ancients?” By no means. “How then,” says one, “did Abraham, how did Job obtain a good report?” Tell me not of them that are rich, but of them that serve riches. Since Job also was rich, but he served not mammon, but possessed it and ruled over it, and was a master, not a slave. Therefore he so possessed all those things, as if he had been the steward of another man's goods; not only not extorting from others, but even giving up his own to them that were in need. And what is more, when he had them they were no joy to him: so he also declared, saying, “If I did so much as rejoice when my wealth waxed great:” wherefore neither did he grieve when it had gone. But they that are rich are not now such as he was, but are rather in a worse condition than any slave, paying as it were tribute to some grievous tyrant. Because their mind is as a kind of citadel occupied by the love of money, which from thence daily sends out unto them its commands full of all iniquity, and there is none to disobey. Be not therefore thus over subtle. Nay, for God has once for all declared and pronounced it a thing impossible for the one service and the other to agree. Say not thou, then, “it is possible.” Why, when the one master is commanding you to spoil by violence, the other to strip yourself of your possessions; the one to be chaste, the other to commit fornication; the one to be drunken and luxurious, the other to keep the belly in subjection; the one again to despise the things that are, the other to be rivetted to the present; the one to admire marbles, and walls, and roofs, the other to contemn these, but to honor self-restraint: how is it possible that these should agree?
Now He calls mammon here “a master,” not because of its own nature, but on account of the wretchedness of them that bow themselves beneath it. So also He calls “the belly a god,” not from the dignity of such a mistress, but from the wretchedness of them that are enslaved: it being a thing worse than any punishment, and enough, before the punishment, in the way of vengeance on him who is involved in it. For what condemned criminals can be so wretched, as they who having God for their Lord, do from that mild rule desert to this grievous tyranny, and this when their act brings after it so much harm even here? For indeed their loss is unspeakable by so doing: there are suits, and molestations, and strifes, and toils, and a blinding of the soul; and what is more grievous than all, one falls away from the highest blessings; for such a blessing it is to be God's servant.
Source: Homilies on the Gospel of St. Matthew (New Advent)