5 My dear brother, weigh well the various forms of transgression, and think not that the sins which I have mentioned are less flagrant than that of idolatry. Nay, hear the apostle's view of the matter. “For this ye know,” he writes, “that no whore-monger or unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and of God.” In a general way all that is of the devil savors of enmity to God, and what is of the devil is idolatry, since all idols are subject to him. Yet Paul elsewhere lays down the law in express and unmistakable terms, saying: “Mortify your members, which are upon the earth, laying aside fornication, uncleanness, evil concupiscence and covetousness, which are idolatry, for which things' sake the wrath of God comes.”
Idolatry is not confined to casting incense upon an altar with finger and thumb, or to pouring libations of wine out of a cup into a bowl. Covetousness is idolatry, or else the selling of the Lord for thirty pieces of silver was a righteous act. Lust involves profanation, or else men may defile with common harlots those members of Christ which should be “a living sacrifice acceptable to God.” Fraud is idolatry, or else they are worthy of imitation who, in the Acts of the Apostles, sold their inheritance, and because they kept back part of the price, perished by an instant doom. Consider well, my brother; nothing is yours to keep. “Whosoever he be of you,” the Lord says, “that forsakes not all that he has, he cannot be my disciple.” Why are you such a half-hearted Christian?
Source: Letters (New Advent)