Catechism of the Catholic Church §732 — excerpt
On that day, the Holy Trinity is fully revealed. Since that day, the Kingdom announced by Christ has been open to those who believe in him: in the humility of the flesh and in faith, they already share in the communion of the Holy Trinity.
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: Byzantine liturgy, Pentecost Vespers, Troparion, repeated after communion