§2084
Morals › … › I. "You Shall Worship the Lord Your God and Him Only Shall You Serve"
God makes himself known by recalling his all-powerful loving, and liberating action in the history of the one he addresses: "I brought you out of the land of Egypt, out of the house of bondage." the first word contains the first commandment of the Law: "You shall fear the LORD your God; you shall serve him.... You shall not go after other gods." God's first call and just demand is that man accept him and worship him.
Source: Catechism of the Catholic Church (Vatican.va)