Catechism of the Catholic Church §450 — excerpt
From the beginning of Christian history, the assertion of Christ's lordship over the world and over history has implicitly recognized that man should not submit his personal freedom in an absolute manner to any earthly power, but only to God the Father and the Lord Jesus Christ: Caesar is not "the Lord".
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: GS 10 # 3 · 45 # 2