What Does the
A Practical Catholic Dictionary, p. 32:
Blessed Trinity, the: One and the same God in three divine persons, the Father, the Son, and the Holy Ghost. There are three distinct persons who are one God. Each of these persons is divine because each one is God. They all have one and the same divine nature. The Father is God and the first person of the Blessed Trinity. The Son is God and the second person of the Blessed Trinity. The Holy Ghost is God and the third person of the Blessed Trinity.
Unless (people) keep this Faith whole and undefiled, without doubt (they) shall perish everlastingly. And the Catholic faith is this: we worship one God in Trinity.
Handbook for Todays Catholic, p. 16
The mystery of the trinity is the central doctrine of Catholic faith. Upon it are based all the other teachings of the church
The Catholic Encyclopedia
The term trinity is the term employed to signify the central doctrine of the Christian religion; the truth that in the unity of the Godhead there are three persons. Thus in the words of the Athanasian Creed, the Father is God, the Son is God and the Holy Spirit is God . . .
This, the church teaches, is the revelation regarding the nature of God which Jesus Christ, the Son of God, came upon earth to deliver to the world; and which she (the church) proposes to man as the foundation of her whole dogmatic system.
From the Beginning of the Catholic System
In the establishment of the Catholic Church, the place of Theodosius is second only to that of Constantine. About the beginning of the year 380 he was baptized by the Catholic bishop of Thessalonica, and immediately afterward he issued the following edict:
. . . Let us believe the sole deity of the Father the Son and the Holy Ghost under an equal majesty and a pious Trinity. We authorize the followers of this doctrine to assume the title of Catholic Christians; and as we judge that all others are extravagant madmen we brand them with the infamous name of heretics, and declare that their conventicles shall no longer usurp the respectable appellation of churches. Besides the condemnation of divine justice they must expect to suffer the severe penalties which our authority guided by heavenly wisdom shall think proper to inflict upon them.