Historical note: New heresies (Arainism) arose to attack the nature of Christ as true God in the Trinity. The Nicene Creed did not fully express the understanding of the person of Christ. The Council of Constantinople met in 451 AD and formulated the Athanasian Creed to end this controversy. The Scriptures reveal Christ’s deity as well as his humanity. As our Savior, His full and complete humanity is just as important as his full and complete deity.
Whoever will be saved shall, above all else, hold the catholic* faith. Which faith, except everyone keeps whole and undefiled, without doubt he will perish eternally. And the catholic faith is this, that we worship one God in three persons and three persons in one God, neither confusing 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 coeternal. 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 there are not three uncreated nor three incomprehensibles 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 they are not three Lords but one Lord. For as we are compelled by the Christian truth to acknowledge every person by himself to be both God and Lord, So we cannot by the catholic faith 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 and of the Son, 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 this Trinity none is before or after another; none is greater or less than another; But the whole three persons are coeternal together and coequal, so that in all things, as is aforesaid, the Unity in Trinity and the Trinity in Unity is to be worshiped. He, therefore, that will be saved is compelled thus to think of the Trinity.
Furthermore, it is necessary to everlasting salvation that we also believe faithfully 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 reasonable 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 the manhood into God; One altogether, not by confusion of substance but by unity of person.
For as the reasonable 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, and he sits at the right hand of the Father, God Almighty, from whence he will come to judge the living and the dead. At whose coming all men will rise again with their bodies and will give an account of their own works.
And they that have done good will go into life everlasting; and they that have done evil, into everlasting fire. This is the catholic faith which, except a man believes faithfully and firmly, he cannot be saved.
* catholic (universal)