Since, then, God knows and at the same time observes everything about us, let us have a faith that is sure and stable. And let us rid ourselves of the shame coming from duplicity, remembering what the blessed Paul writes: "Therefore, my brothers, be steadfast and immovable, always abounding in God's work." (1Cor 15:58)
Come, then, while offering to God a genuine faith as a spiritual sacrifice, let us say from a heart free of suspicion, "Behold us, we will be yours, because you are the Lord our God. " (Jer 3:22) And again, "Lord our God, take possession of us. Lord, we know not any other beside you. Your name we name." (Isa 26:13)
For those who choose to think rightly must believe that the God who is over all and through all is the one and only God. He did not bring himself into being, nor of course did he receive that from another. But being and existing always, he is eternally before every age and time. Incorruptible and indestructible, "dwelling in light inaccessible," (1 Tim 6:16) the source of wisdom and life, he is conceived of as BEING, and is, by nature, that which is good in itself, the root of all power. And while he is known to be, it is not known what HE is by nature. For that must be our attitude, as the Saviour's disciple said in speaking as follows: "Those who approach God must believe that HE IS, and that he rewards those who seek him." (Heb 11:6) But they must not search further.
Do not doubt within yourself, my friend, nor allow yourself to be pushed into a perilous curiosity by arguments springing from ignorance, so that you venture to say: I have been initiated and have believed, and have been taught to worship the one God. But why should I not learn the nature of the one adored?
Away with you and your ill-advised attempt to investigate matters which surpass the mind and its reason! "What, after all, is human thought in comparison to that ineffable and inexpressible nature? Who would be of such intelligence as to be able to measure that beauty? ...
The Church's message is simple. We have been baptised, that is, in the Father and the Son and the Holy Spirit. And believing that the Holy Trinity is co-essential, we worship one godhead in it, thanking God the Father for having sent from heaven his own Son for the sake of our salvation and life, the Son born of a woman, (Gal 4:4) the Son who put on our likeness and truly became a human being, in order that, having triumphed over the Principalities and Powers, (Col 2:15) he might nail to his own cross, as is written, the bond that stood against us, (Col 2:14) and might render us pure, freed from all guilt, once he had washed us of the defilement of our failings of the past.
His purpose was also that he might preach "also to the spirits in hell, who formerly did not obey," (1Pet 3:19-20) as is written, in order thus at last to abolish death, the enemy of all, once he had been raised from the dead, and indeed, when he had opened the gates above to those on earth, to make the former runaway a citizen of heaven. For he will come, he will come indeed when the time arrives, and, as he himself said, (Jn 14:3, 1 Thes 4:16-17) he will take with him all of us who are distinguished by right faith and illustrious for an evangelical way of life.
Saint Cyril of Alexandria: Festal Letter twelve (424 AD) in St. Cyril of Alexandria Festal letters 1-12, The Catholic University of America Press