Let every one, then, see to it that he esteem this commandment great and high above all things, and do not regard it as a joke. 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.
Martin Luther's Large Catechism