Catechism of the Catholic Church §1296 — excerpt
Christ himself declared that he was marked with his Father's seal. Christians are also marked with a seal: "It is God who establishes us with you in Christ and has commissioned us; he has put his seal on us and given us his Spirit in our hearts as a guarantee."
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.
Whole chapters cited: Ephesians 4