Gregory of Nazianzus (c. 329–390):

Keep I pray you the good deposit,
by which I live and work, and
which I desire to have as the companion of my departure;
with which I endure all that is so distressful, and despise all delights;

the confession of the Father and the Son and the Holy Ghost.

This I commit unto you today;
with this I will baptize you and make you grow.
This I give you to share, and to defend all your life,

the one Godhead and power, found in the three in unit, and comprising the three separately;
not unequal, in substances or natures, neither increased nor diminished by superiorities nor inferiorities;
in every respect equal, in every respect the same;
just as the beauty and the greatness of the heavens is one;
the infinite conjunction of three infinite ones, each God when considered in himself;
as the Father, so the Son;
as the Son, so the Holy Spirit;

the three one God when contemplated together;
each God because consubstantial;
one God because of the monarchia.

No sooner do I conceive of the one than I am illumined by the splendor of the three;
no sooner do I distinguish them than I am carried back to the one. 

When I think of anyone of the three I think of him as the whole, and my eyes are filled, and the greater part of what I am thinking escapes me.
I cannot grasp the greatness of that one so as to attribute a greater greatness to the rest.

When I contemplate the three together, I see but one torch, and cannot divide or measure out the undivided light.

—Gregory Nazianzen, Orations 40.41.