The Athanasian Creed (c. 5th century A.D.)

Whoever wishes to be saved must, above all else, hold the true Christian faith. Whoever does not keep it whole and undefiled will without doubt perish for eternity.
This is the true Christian faith, that we worship one God in three persons and three persons in one God
Without confusing the persons or dividing the divine substance.
For the Father is one person, the Son is another, and the Holy Spirit is still another,
But there is one Godhead of the Father and of the Son and of the Holy Spirit, equal in glory and coequal in majesty.
What the Father is, that is the Son and that is the Holy Spirit:
The Father is uncreated, the son is uncreated, the Holy Spirit is uncreated;
The Father is unlimited, the Son is unlimited, the Holy Spirit is unlimited;
The Father is eternal, the Son is eternal, the Holy Spirit is eternal;
And yet they are not three eternals but one eternal,
Just as there are not three who are uncreated and who are unlimited, but there is one who is uncreated and unlimited.
Likewise the Father is almighty, the Son is almighty, the Holy Spirit is almighty,
And yet there are not three who are almighty but there is one who is almighty.
So the Father is God, the Son is God, the Holy Spirit is God,
And yet they are not three Gods but one God.
So the Father is Lord, the Son is Lord, the Holy Spirit is Lord,
And yet they are not three Lords but one Lord.
For just as we are compelled by Christian truth to acknowledge each person by himself to be God and Lord,
So we are forbidden by the Christian religion to say that there are three Gods or three Lords.
The Father was neither made nor created nor begotten by anybody.
The Son was not made or created, but was begotten by the Father.
The Holy Spirit was not made or created or begotten, but proceeds from the Father and the Son.
Accordingly there is one Father and not three Fathers, one Son and not three Sons, one Holy Spirit and not three Holy Spirits.
And among these three persons none is before or after another, none is greater or less than another,
But all three persons are coequal and coeternal, and accordingly, as has been stated above, three persons are to be worshiped in one Godhead and one God is to be worshiped in three persons.
Whoever wishes to be saved must think thus about the Trinity.
It is also necessary for eternal salvation that one faithfully believe that our Lord Jesus Christ became man,
For this is the right faith, that we believe and confess that our Lord Jesus Christ, the Son of God, is at once God and man:
He is God, begotten before the ages of the substance of the Father, and he is man, born in the world of the substance of his mother,
Perfect God and perfect man, with reasonable soul and human flesh,
Equal to the Father with respect to his Godhead and infereior to the Father with respect to his manhood.
Although he is God and man, he is not two Christs but one Christ:
One, that is to say, not by changing the Godhead into flesh but by taking on the humanity into God,
One, indeed, not by confusion of substance but by unity in one person.
For just as the reasonable soul and the flesh are one man, so God and man are one Christ,
Who suffered for our salvation, descended into hell, rose from the dead,
Ascended into heaven, is seated on the right hand of the Father, whence he shall come to judge the living and the dead.
At his coming all men shall rise with their bodies and give an account of their own deeds.
Those who have done good will enter eternal life, and those who have done evil will go into everlasting fire.
This is the true Christian faith. Unless a man believe this firmly and faithfully, he cannot be saved.