What is it to have a god? Or, what is one’s god? Answer: To whatever we look for any good thing and for refuge in every need, that is what is meant by “god”…it is the trust and faith of the heart, nothing else, that make both God and an idol…. To whatever you give your heart and entrust your being, that…is really your god…. Many a person imagines that he has God and everything he needs, provided he has money and property. He relies upon these, boasts about them, and feels…immovably secure…. But look, he too has a god, named mammon, that is the money and property to which he has given his whole heart. Mammon is the world’s favorite idol. One who has money and property has a sense of security and feels…happy and fearless…. On the other hand, one who has nothing is as insecure and anxiety-ridden as if he had never heard of God…. Similarly, one who congratulates himself on his great learning, intelligence, power, special advantages, family connections, and honor and trusts in them also has a god, only not the one true God. The evidence for this appears when people are arrogant, secure, and proud because of such possessions, but desperate when they lack them or lose them. I repeat, to have a god means to have something on which one’s heart depends entirely.
So now you can easily understand what it is and how much it is that this [first] commandment requires. It requires that man’s whole heart and all his confidence be given to God alone and no one else…. The true worship and service of God, the kind that pleases Him and which He also commands on pain of everlasting wrath…takes place when your heart directs all its trust and confidence only toward God and does not let itself be torn away from Him; it consists in risking everything on earth for Him and abandoning it all for His sake. You can easily judge how, in contrast to this, the world practices nothing but false worship and idolatry…. Everyone has set up for himself some particular god to which he looks for benefits, help, and comfort…. For idolatry does not consist simply in setting up an image and worshiping it; it takes place primarily in the heart, which looks elsewhere than to the one God, seeks help and comfort in created things…. Besides this there is also that false worship, that height of idolatry, which…involves…those who seek comfort and salvation in their own works and presume to capture heaven by putting God under the pressure of an obligation…, they wish to earn everything themselves or merit it by works…. What is this but turning God into an idol, into a plaster image, while the worshiper actually is setting himself up as his own god. However:…We are to trust in God alone, look to Him, and expect to receive nothing but good things from Him…. Question and explore your own heart thoroughly, and you will find out if it embraces God alone or not. Do you have it in your heart to expect nothing but good things from God, especially when you are in trouble and in need? And does your heart in addition give up and forsake everything that is not God? Then you have the one true God. On the other hand, is your heart attached to and does it rely on something else, from which you hope to receive more good and more help than from God? And when things go wrong, do you, instead of fleeing to Him, flee from Him? Then you have another god, a false god, [an idol].
– Martin Luther, from Luther’s Larger Catechism.