5 So then those are the true riches, which when we have them, we cannot lose. And lest haply you should fear a thief because of them, they will be there where none can take them away. Hear your Lord, “Lay up for yourselves treasures in heaven, where no thief approaches.” Then will they be riches, when you have removed them hence. As long as they are in the earth, they are not riches. But the world calls them riches, iniquity calls them so. God calls them therefore the mammon of iniquity, because iniquity calls them riches.
Hear the Psalm, “O Lord, deliver me out of the hand of strange children, whose mouth has spoken vanity, and their right hand is a right hand of iniquity. Whose sons are as new plants, firmly rooted from their youth. Their daughters decked out, adorned round about after the similitude of a temple. Their storehouses full, flowing out from this into that. Their oxen fat, their sheep fruitful, multiplying in their goings forth. There is no breach of wall, nor going forth, no crying out in their streets.” Lo, what sort of happiness the Psalmist has described: but hear what is the case with them whom he has set forth as children of iniquity.
“Whose mouth has spoken vanity, and their right hand is a right hand of iniquity.” Thus has he set them forth, and said that their happiness is only upon the earth. And what did he add? “They are happy the people that has these things.” But who called them so? “Strange children,” aliens from the race, and belonging not to the seed of Abraham: they “called the people happy that has these things.” Who called them so? “They whose mouth has spoken vanity.” It is a vain thing then to call them happy who have these things. And yet they are called so by them, “whose mouth has spoken vanity.” By them the “mammon of iniquity” of the Gospel is called riches.
Source: Sermons on Selected Lessons of the New Testament (New Advent)