Matt. VI. 24.
“No man can serve two masters, for either he will hate the one and love the other, or else he will hold to one and despise the other.”
Do you see how by degrees He withdraws us from the things that now are, and at greater length introduces what He has to say, touching voluntary poverty, and casts down the dominion of covetousness?
For He was not contented with His former sayings, many and great as they were, but He adds others also, more and more alarming.
For what can be more alarming than what He now says, if indeed we are for our riches to fall from the service of Christ? Or what more to be desired, if indeed, by despising wealth, we shall have our affection towards Him and our charity perfect? For what I am continually repeating, the same do I now say likewise, namely, that by both kinds He presses the hearer to obey His sayings; both by the profitable, and by the hurtful; much like an excellent physician, pointing out both the disease which is the consequence of neglect, and the good health which results from obedience.
See, for instance, what kind of gain He signifies this to be, and how He establishes the advantage of it by their deliverance from the contrary things. Thus, “wealth,” says He, “hurts you not in this only, that it arms robbers against you, nor in that it darkens your mind in the most intense degree, but also in that it casts you out of God's service, making you captive of lifeless riches, and in both ways doing you harm, on the one hand, by causing you to be slaves of what you ought to command; on the other, by casting you out of God's service, whom, above all things, it is indispensable for you to serve.” For just as in the other place, He signified the mischief to be twofold, in both laying up here, “where moth corrupts,” and in not laying up there, where the watch kept is impregnable; so in this place, too, He shows the loss to be twofold, in that it both draws off from God, and makes us subject to mammon.
But He sets it not down directly, rather He establishes it first upon general considerations, saying thus; “No man can serve two masters:” meaning here two that are enjoining opposite things; since, unless this were the case, they would not even be two. For so, “the multitude of them that believed were of one heart and of one soul,” and yet were they divided into many bodies; their unanimity however made the many one.
Then, as adding to the force of it, He says, “so far from serving, he will even hate and abhor:” “For either he will hate the one,” says He, “and love the other, or else he will hold to the one and despise the other.” And it seems indeed as if the same thing were said twice over; He did not however choose this form without purpose, but in order to show that the change for the better is easy. I mean, lest you should say, “I am once for all made a slave; I am brought under the tyranny of wealth,” He signifies that it is possible to transfer one's self, and that as from the first to the second, so also from the second one may pass over to the first.
Source: Homilies on the Gospel of St. Matthew (New Advent)