Third-century Latin church father Novatian of Rome is not well-known today, but was an important figure in his time. He was an anti-pope, meaning he opposed the bishop of Rome, and was elected as a rival bishop. This caused a lot of controversy, which is not within the scope of this post to explore.
Novatian is noteworthy regardless of other shortcomings he may have had for his small contribution to trinitarian doctrine, in his treatise Concerning the Trinity. In it he elucidates his understanding of classical trinitarianism. In doing so he argues for both the divinity of Christ and the doctrine that the one God is the person of the Father, and defends the biblical truth that the Father is the one God by showing it is compatible with the doctrine of the Son’s divinity.
This can be seen from these quotes:
“Thus God the Father, the Founder and Creator of all things, who only knows no beginning, invisible, infinite, immortal, eternal, is one God; to whose greatness, or majesty, or power, I would not say nothing can be preferred, but nothing can be compared; of whom, when He willed it, the Son, the Word, was born, who is not received in the sound of the stricken air, or in the tone of voice forced from the lungs, but is acknowledged in the substance of the power put forth by God, the mysteries of whose sacred and divine nativity neither an apostle has learnt, nor prophet has discovered, nor angel has known, nor creature has apprehended.” A Treatise of Novatian Concerning the Trinity, Chapter XXXI.
“Assuredly God proceeding from God, causing a person second to the Father as being the Son, but not taking from the Father that characteristic that He is one God.” A Treatise of Novatian Concerning the Trinity, Chapter XXXI.
Notice here how explicitly he shows his belief that the Father is the one God- to be the one God is a characteristic special to the Father, not taken from Him by virtue of His having His Son as another distinct divine person.
“But now, whatever He [Christ] is, He is not of Himself, because He is not unborn; but He is of the Father, because He is begotten, whether as being the Word, whether as being the Power, or as being the Wisdom, or as being the Light, or as being the Son; and whatever of these He is, in that He is not from any other source, as we have already said before, than from the Father, owing His origin to His Father, He could not make a disagreement in the divinity by the number of two Gods, since He gathered His beginning by being born of Him who is one God.” A Treatise of Novatian Concerning the Trinity, Chapter XXXI.
“Thus making Himself obedient to His Father in all things, although He also is God, yet He shows the one God the Father by His obedience, from whom also He drew His beginning.” A Treatise of Novatian Concerning the Trinity, Chapter XXXI.
Here we last see Novatian answer the objection that the Son constitutes a second God because He is another supreme head over all creation, as the Father is (since “God” indicates headship, such as in the phrase “the Lord is my God”). Novatian’s response, again characteristic of the orthodox fathers of the early church, is that since the Son is subject to the Father in everything, eternally obedient to Him in filial love, the Son does not constitute a rival headship over the universe, so as to make a second God. Rather, the Son Himself is eternally subject to the headship of the Father, the one God (1 Cor 11:3).