I. "IN THE NAME OF THE FATHER AND OF THE SON AND OF THE HOLY SPIRIT"
This paragraph expounds briefly (I) how the mystery of the Blessed Trinity was revealed, (II) how the Church has articulated the doctrine of the faith regarding this mystery, and (III) how, by the divine missions of the Son and the Holy Spirit, God the Father fulfils the "plan of his loving goodness" of creation, redemption and sanctification.
Source: Catechism of the Catholic Church (Vatican.va)