Catechism of the Catholic Church

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. (2057, 398)