Catechism of the Catholic Church §223 — excerpt
It means coming to know God's greatness and majesty: "Behold, God is great, and we know him not." Therefore, we must "serve God first".
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: St. Joan of Arc