Catechism of the Catholic Church §443 — excerpt
Peter could recognize the transcendent character of the Messiah's divine sonship because Jesus had clearly allowed it to be so understood. To his accusers' question before the Sanhedrin, "Are you the Son of God, then?"
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.