Martin Luther’s Large Catechism begins with a shrewd reflection on the first commandment:

“You are to have no other gods.”

That is, you are to regard me alone as your God. What does this mean, and how is it to be understood? What does “to have a god” mean, or what is God?

Answer: A “god” is the term for that to which we are to look for all good and in which we are to find refuge in all need. Therefore, to have a god is nothing else than to trust and believe in that one with your whole heart. As I have often said, it is the trust and faith of the heart alone that make both God and an idol. If your faith and trust are right, then your God is the true one. Conversely, where your trust is false and wrong, there you do not have the true God. For these two belong together, faith and God. Anything on which your heart relies and depends, I say, that is really your God.

—Martin Luther, Large Catechism, “[The First Part: The Ten Commandments],” The Book of Concord: The Confessions of the Evangelical Lutheran Church (ed. Robert Kolb and Timothy J. Wengert; trans. Charles Arand, et al.; Minneapolis: Fortress, 2000), 386.

Luther proceeds to elaborate further on the relationship between idolatry and trust (386–92). You can read it via Google Books.