Now the catholic faith is 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, another of the Holy Spirit. But the Godhead of the Father, of the Son, and of the Holy Spirit, is 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 infinite, the Son infinite, and the Holy Spirit infinite;
the Father eternal, the Son eternal, and the Holy Spirit eternal.
And yet not three eternals but one eternal, as also not three infinites, nor three uncreated, but one uncreated, and one infinite.
So, likewise, the Father is almighty, the Son almighty, and the Holy Spirit almighty;
and yet not three almighties but one almighty.
So the Father is God, the Son God, and the Holy Spirit God;
and yet not three Gods but one God.
So the Father is Lord, the Son Lord, and the Holy Spirit Lord;
and yet not three Lords but one Lord. For like as we are compelled by Christian truth to acknowledge every Person by Himself to be both God and Lord;
so are we forbidden by the catholic religion to say, there be 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 the Son, not made nor created nor begotten but proceeding.
So there is one Father not three Fathers, one Son not three Sons, and Holy Spirit not three Holy Spirits. And in this Trinity there is nothing before or after, nothing greater or less, but the whole three Persons are coeternal together and coequal.
So that in all things, as is aforesaid, the trinity in Unity and the Unity in Trinity is to be worshipped. He therefore who wills to be in a state of salvation, let him think thus of the Trinity.
Actually, it may not have been him who came up with it. According to The Catholic Encyclopedia, the origin and authorship of the Athanasian Creed is unclear.
For the past (three) hundred years the authorship of this summary of Catholic Faith and the time of its appearance have furnished an interesting problem to ecclesiastical antiquarians. Until the seventeenth century, the "Quicunque vult", as it is sometimes called, from its opening words, was thought to be the composition of the great Archbishop of Alexandria whose name it bears. In the year 1644, Gerard Voss, in his "De Tribus Symbolis", gave weighty probability to the opinion that St. Athanasius was not its author. His reasons may be reduced to the two following:
Firstly, no early writer of authority speaks of it as the work of this doctor; and secondly, its language and structure point to a Western, rather than to an Alexandrian, origin.
Most modern scholars agree in admitting the strength of these reasons, and hence this view is the one generally received today. Whether the Creed can be ascribed to St. Athanasius or not, and most probably it cannot, it undoubtedly owes it existence to Athanasian influences, for the expressions and doctrinal colouring exhibit too marked a correspondence, in subject-matter and in phraseology, with the literature of the latter half of the fourth century and especially with the writings of the saint, to be merely accidental. These internal evidences seem to justify the conclusion that it grew out of several provincial synods, chiefly that of Alexandria, held about the year 361, and presided over by St. Athanasius.