I. "IN THE NAME OF THE FATHER AND OF THE SON AND OF THE HOLY SPIRIT"
Christians are baptized "in the name of the Father and of the Son and of the Holy Spirit" Before receiving the sacrament, they respond to a three-part question when asked to confess the Father, the Son and the Spirit: "I do." "The faith of all Christians rests on the Trinity."
Source: Catechism of the Catholic Church (Vatican.va)