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

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