Idolatry does not consist merely of erecting an image and praying to it, but it is primarily a matter of the heart, which fixes its gaze upon other things and seeks help and consolation from creatures, saints, or devils. It neither cares for God nor expects good things from God sufficiently to trust that God wants to help, nor does it believe that whatever good it encounters comes from God.

Kirsi I. Stjerna, “The Large Catechism of Dr. Martin Luther,” in Word and Faith, ed. Hans J. Hillerbrand, Kirsi I. Stjerna, and Timothy J. Wengert, vol. 2, The Annotated Luther (Minneapolis, MN: Fortress Press, 2015), 302–303.