Please note that the term "catholic" with the lower case 'c' is not a reference to the Roman Catholic Church, but is a reference to the universal Christian faith, since that is how the term was originally used.
Whosoever will be saved, before all things it is necessary that he hold the catholic faith. Which faith except everyone do keep whole and undefiled, without doubt he shall perish everlastingly. And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity, neither confounding the persons, nor dividing the substance.
For there is one Person of the Father, another of the Son, and another of the Holy Spirit. But the godhead of the Father, of the Son, and of the Holy Spirit, is all one, the glory equal, the majesty co-eternal.
Such as the Father is, such is the Son, and such is the Holy Spirit. The Father uncreated, the Son uncreated, and the Holy Spirit uncreated. The Father incomprehensible, the Son incomprehensible, and the Holy Spirit incomprehensible.
The Father eternal, the Son eternal, and the Holy Spirit eternal. And yet they are not three eternals, but one Eternal.
As also there are not three incomprehensibles, nor three uncreated, but one Uncreated, and one Incomprehensible. So likewise the Father is Almighty, the Son Almighty, and the Holy Spirit Almighty. And yet they are not three almighties, but one Almighty.
So the Father is God, the Son is God, and the Holy Spirit is God. And yet they are not three gods, but one God.
So likewise the Father is Lord, the Son Lord, and the Holy Spirit Lord. And yet not three lords, but one Lord.
For as we are compelled by the Christian verity to acknowledge each Person by Himself to be both God and Lord, so we are also forbidden by the catholic religion to say that there are three gods or three lords.
The Father is made of none, neither created, nor begotten. The Son is of the Father alone, not made, nor created, but begotten. The Holy Spirit is of the Father, neither made, nor created, nor begotten, but proceeding.
So there is one Father, not three fathers; one Son, not three sons; one Holy Spirit, not three holy spirits.
And in the Trinity none is before or after another; none is greater or less than another, but all three Persons are co-eternal together and co-equal. So that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshipped.
He therefore that will be saved must think thus of the Trinity.
Furthermore, it is necessary to everlasting salvation that he also believe rightly the Incarnation of our Lord Jesus Christ. For the right faith is, that we believe and confess, that our Lord Jesus Christ, the Son of God, is God and man; God, of the substance of the Father, begotten before the worlds; and man of the substance of his mother, born in the world; perfect God and perfect man, of a rational soul and human flesh subsisting. Equal to the Father, as touching His godhead; and inferior to the Father, as touching His manhood; who, although He is God and man, yet he is not two, but one Christ; one, not by conversion of the godhead into flesh but by taking of the manhood into God; one altogether; not by confusion of substance, but by unity of person. For as the rational soul and flesh is one man, so God and man is one Christ; who suffered for our salvation, descended into hell, rose again the third day from the dead. He ascended into heaven, He sits at the right hand of the Father, God Almighty, from whence He will come to judge the quick and the dead. At His coming all men will rise again with their bodies and shall give account for their own works. And they that have done good shall go into life everlasting; and they that have done evil into everlasting fire.
This is the catholic faith, which except a man believe faithfully, he cannot be saved.