“If you have a heart that can expect of Him nothing but what is good – especially in need and distress – and a heart that also renounces and forsakes everything that is not God, then you have the only true God. If, on the contrary, your heart clings to anything else from which it expects more good and help than from God, and if your heart does not take refuge in Him but flees from Him when in trouble, then you have an idol, another god.”
Martin Luther, Large Catechism, Explanation on the First Commandment “You shall have no other gods”.
Spiritual Encouragement: A Christian Journey 