47 Then, further, the statement which follows, “No man can serve two masters,” is to be referred to this very intent, as He goes on to explain, saying: “For either he will hate the one, and love the other; or else he will submit to the one, and despise the other.” And these words are to be carefully considered; for who the two masters are he immediately shows, when He says, “You cannot serve God and mammon.” Riches are said to be called mammon among the Hebrews. The Punic name also corresponds: for gain is called mammon in Punic. But he who serves mammon certainly serves him who, as being set over those earthly things in virtue of his perversity, is called by our Lord the prince of this world. A man will therefore “either hate” this one, “and love the other,” i.e. God; “or he will submit to the one, and despise the other.” For whoever serves mammon submits to a hard and ruinous master: for, being entangled by his own lust, he becomes a subject of the devil, and he does not love him; for who is there who loves the devil? But yet he submits to him; as in any large house he who is connected with another man's maid servant submits to hard bondage on account of his passion. even though he does not love him whose maid-servant he loves.
48. But “he will despise the other,” He has said; not, he will hate. For almost no one's conscience can hate God; but he despises, i.e. he does not fear Him, as if feeling himself secure in consideration of His goodness. From this carelessness and ruinous security the Holy Spirit recalls us, when He says by the prophet, “My son, do not add sin upon sin, and say, The mercy of God is great;” and, “Do you not know that the patience of God invites you to repentance?” For whose mercy can be mentioned as being so great as His, who pardons all the sins of those who return, and makes the wild olive a partaker of the fatness of the olive? And whose severity as being so great as His, who spared not the natural branches, but broke them off because of unbelief? But let not any one who wishes to love God, and to beware of offending Him, suppose that he can serve two masters; and let him disentangle the upright intention of his heart from all doubleness: for thus he will think of the Lord with a good heart, and in simplicity of heart will seek Him.
Source: Our Lord's Sermon on the Mount (New Advent)