The Athanasian Creed is dated from A.D. 361. It is named after Athanasius, but it is unlikely he wrote it.
It has some differences from the Nicene Creed, which are addressed on the Trinity page on this site. Those differences developed during the 4th century when the debate with the Arians (named after Arius of Alexandria) was raging.
Icon of Athanasius
The Council of Nicea was supposed to have settled the debate with the Arians, but it did not. The Arian view of the Trinity experienced a revival during the reign of Constantius, son of Constantine the Great, who—more often than not—supported the Arians and attempted to install Arian bishops on several occasions.
Modalism (the belief that there was no Trinity but that God was one Person acting out the three roles of Father, Son, and Holy Spirit) was in the Church long before Arius made his views popular. Arianism was offensive to both Nicene and modalist bishops, and they banded together to overthrow it.
The result was a view of the Trinity that united Nicene and modalist beliefs. It is this version of the Trinity that is preserved in the Athanasian Creed.
The Athanasian Creed continues to represent the "orthodox" view of the Trinity to this day. The Nicene or Apostles Creed is quoted in churches today, but it is not believed with its original meaning.
Whosoever will be saved, before all things it is necessary that he hold the catholic [i.e., "universal," not Roman Catholic] faith; Unless everyone keeps this faith whole and undefiled, then without doubt he shall perish everlastingly.
The catholic faith is this: we worship one God in Trinity and Trinity in Unity, neither confounding the persons nor dividing the substance
There is one person of the Father, another of the Son, and another of the Holy Spirit, but the divinity 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 also there are not three uncreated nor three incomprehensible, 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.
Just as we are compelled by Christian truth to acknowledge every Person by himself to be God and Lord, so we are forbidden by the catholic religion to say there are three Gods or three Lords.
The Father is made by no one, 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.
In this Trinity none is before or after another, none is greater or less than another, but all three persons are coeternal and coequal, so that in all things, as said before, the Unity in Trinity and the Trinity in Unity is to be worshipped.
He therefore that will be saved must think of the Trinity in this way.
Jesus Christ, the Son of God
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 substance of His mother, born in the world; perfect God and perfect man, subsisting of a reasonable soul and human flesh; equal to the Father as touching his divinity, and inferior to the Father as touching his manhood, who, although he is God and man, yet he is not two, but one Christ.
He is one, not by conversion of divinity into flesh, but by taking that 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, and rose again the third day from the dead. He ascended into heaven, he sits on the right hand of the Father, God Almighty; from there he shall come to judge the living and the dead.
The Judgment and Conclusion
At his coming all men shall rise again with their bodies and shall give account of their own works. 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.