From the Heidelberg Catechism for the Lord’s Day, September 1

93. How are these [The Ten] Commandments divided?

Into two tables: the first of which teaches, in four commandments, what duties we owe to God; the second, in six, what duties we owe to our neighbor.

94. What does God require in the first Commandment?

That, on peril of my soul’s salvation, I avoid and flee all idolatry, sorcery, enchantments, invocation of saints or of other creatures; and that I rightly acknowledge the only true God, trust in Him alone,  with all humility and patience expect all good from Him only, and love, fear, and honor Him with my whole heart; so as rather to renounce all creatures than to do the least thing against His will.

95. What is idolatry?

Idolatry is to conceive or have something else in which to place our trust instead of, or besides, the one true God who has revealed Himself in His Word.