Before all else, I beg you…hold fast to the confession of the Father and the Son and the Holy Spirit.

[...] I give you to share, and to defend all your life, the one Godhead and power, found in the Three in unity, and comprising the Three separately, not unequal in substance or nature, neither increased nor diminished by superiorities or inferiorities; in every respect equal, in every respect the same.

[...] This is the infinite conjunction of three infinite Ones, each is God when considered in Himself; as the Father, so the Son; as the Son, so the Holy Spirit.

No sooner do I conceive of the One than I am illumined by the splendour of the Three; no sooner do I distinguish them than I am carried back to the One.

When I think of any One of the Three I think of him as the Whole, and my eyes are filled, and the greater part of what I am thinking of 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.

I should like to call the Father the greater, because from him flows both the equality and the being of the equals, but I am afraid to use the word ‘origin’, lest I should make him the origin of inferiors and thus insult him by creating a precedence of honour.

For to put down those who are from him gives no glory to him as the source.

I fear you should take hold of this word ‘greater’, and divide the nature, using the word greater in all senses, although in reality it does not apply to the nature but only to origination.

For in the consubstantial Persons there is nothing greater or less in terms of substance.

Are you afraid of being reproached with Tritheism? Just accept this one good thing, the unity in the Three, and leave me to fight the battle.

[...] What need have I any more of speech? It is the time for teaching, not for controversy. I protest before God and the elect angels: be baptized in this faith.

If there is any other writing on your heart than that which my teaching demands, come and have the writing changed; I am no unskilled calligrapher of these truths.

I write that which is written upon my own heart; I teach that which I have been taught, and I have kept it from the beginning even up to the time of these grey hairs.

Gregory Nazianzen (c.330-390): Oration 40, 41-44;  from the Monastic Office of Vigils, Trinity Sunday, Year 2.