Many a one thinks that he has God and everything in abundance when he has money and possessions; he trusts in them and boasts of them with such firmness and assurance as to care for no one. Lo, such a man also has a god, Mammon by name, i.e., money and possessions, on which he sets all his heart, and which is also the most common idol on earth. He who has money and possessions feels secure, and is joyful and undismayed as though he were sitting in the midst of Paradise. On the other hand, he who has none doubts and is despondent, as though he knew of no God… So, too, whoever trusts and boasts that he possesses great skill, prudence, power, favor, friendship, and honor has also a god, but not this true and only God.
This appears again when you notice how presumptuous, secure, and proud people are because of such possessions, and how despondent when they no longer exist or are withdrawn. Therefore I repeat that the chief explanation of this point is that to have a god is to have something in which the heart entirely trusts… Thus it is with all idolatry; for it consists not merely in erecting an image and worshiping it, but rather in the heart, which stands gaping at something else, and seeks help and consolation from creatures, saints, or devils, and neither cares for God, nor looks to Him for so much good as to believe that He is willing to help, neither believes that whatever good it experiences comes from God. Ask and examine your heart diligently, and you will find whether it cleaves to God alone or not. If you have a heart that can expect of Him nothing but what is good, especially in want and distress, and that, moreover, renounces and forsakes everything that is not God, then you have the only true God. If, on the contrary, it cleaves to anything else, of which it expects more good and help than of God, and does not take refuge in Him, but in adversity flees from Him, then you have an idol, another god.
Commentary on the First of the Ten Commandments.