Cyril of Jerusalem is a notable fourth century theologian and church father, best known today for his Catachetical Lectures. This nicene-era archbishop of Jerusalem’s lectures provide us with a clear elucidation of classical trinitarianism, in which we also find strong apolegetics against both Arianism and modalism, both of which threatened the church of his day. Cyril also presided over the second ecumenical council at Constantinople in 381, by which the original Nicene Creed was confirmed and expanded to more explicitly defend the divinity of the Holy Spirit.
Like the Nicene Creed, Cyril very explcitly taught that the “one God” of the Christian faith is the person of the Father in particular. We see that doctrine greatly emphaized by him in the following quotes:
“Further, do thou neither separate the Son from the Father, nor by making a confusion believe in a Son-Fatherhood; but believe that of One God there is One Only-begotten Son, who is before all ages God the Word; not the uttered word diffused into the air, nor to be likened to impersonal words; but the Word the Son, Maker of all who partake of reason, the Word who heareth the Father, and Himself speaketh.” On the Ten Points of Doctrine (Lecture IV)
“For there is One God, the Father of Christ; and One Lord Jesus Christ, the Only-begotten Son of the Only God; and One Holy Ghost…” On the Ten Points of Doctrine (Lecture IV)
“Of God as the sole Principle we have said enough to you yesterday: by “enough” I mean, not what is worthy of the subject, (for to reach that is utterly impossible to mortal nature), but as much as was granted to our infirmity. I traversed also the bye-paths of the manifold error of the godless heretics: but now let us shake off their foul and soul-poisoning doctrine, and remembering what relates to them, not to our own hurt, but to our greater detestation of them, let us come back to ourselves, and receive the saving doctrines of the true Faith, connecting the dignity of Fatherhood with that of the Unity, and believing In One God the Father: for we must not only believe in one God; but this also let us devoutly receive, that He is the Father of the Only-begotten, our Lord Jesus Christ.” The Father (Lecture VII)
“But let us adopt the godly doctrine of our Faith, worshipping one God the Father of the Christ…” The Father (Lecture VII)
“There is One God, the Father, Lord of the Old and of the New Testament: and One Lord, Jesus Christ, who was prophesied of in the Old Testament, and came in the New; and One Holy Ghost, who through the Prophets preached of Christ, and when Christ was come, descended, and manifested Him.” On the Article, And In One Holy Ghost, the Comforter, Which Spake In the Prophets (Lecture XVI)
“The Father through the Son, with the Holy Ghost, is the giver of all grace; the gifts of the Father are none other than those of the Son, and those of the Holy Ghost; for there is one Salvation, one Power, one Faith; One God, the Father; One Lord, His only-begotten Son; One Holy Ghost, the Comforter. ” On the Article, And In One Holy Ghost, the Comforter, Which Spake In the Prophets (Lecture XVI)
There is much we could say about these quotes. It is firstly noteworthy that these all come from lectures given to new believers preparing to be baptised; they were intended to be doctrinal milk for spiritual children. The inclusion of this doctrine in these lectures then shows us the elementary nature of this doctrine in the eyes of Cyril. For Cyril, the truth that the one God of the Christian faith is the Father of Christ was not something esoteric, only to be discussed by theologians in ivory towers. Rather, it was regarded by him as among the most fundamental teachings of the Christian faith, a truth to be understood by every believer, from the advanced scholar to the illiterate farmer.
Secondly, it is noteworthy that Cyril is among the last church fathers to teach this doctrine in his writings. Prior to him we can observe a long line of theologians all teaching this going back to the apostles and scripture itself (see here: https://nicenefaith.wordpress.com/2017/03/08/i-believe-in-one-god-the-father-almighty/ ). In his lectures, we can observe the influence of earlier church fathers like Irenaeus on his theology. Yet after Cyril, this author knows of no other church father who clearly elucidates this doctrine.
A year after Cyril presided over an ecumenical council confirming the Nicene decision in the East, another council was held at Rome, wherein we see the first signs of semi-modalism already having taken root in the Latin church. Within less than twenty years, Augustine would write his volumes on the Trinity popularizing semi-modalism, a view antithetical to that of Cyril and the earlier fathers he followed. In the generation following Cyril we see a spectrum of views as the church gradually abandoned a classical articulation of the Trinity.