Second century church father Athenagoras of Athens is known best for his work Plea for the Christians, an apologetical work in which he urges the Roman authorities to stop unjustly persecuting Christians. Not much is known of his life, but he seems to have been an Athenian philosopher who converted to Christianity, and afterwards wrote in its defense.
Athenagoras stands as another witness to the view of the orthodox church fathers of the second century, the view that the one God of the Christian faith is the person of the Father in particular; not the entire Trinity as a whole. The one God, after all, according to scripture, is a person, not some abstract divinity without real existence. The Trinity, however, is not a person, and certainly not the person of the one God. Rather, scripture teaches that the person of the one God is the same person Who is the Father of the Lord Jesus Christ, His eternal Logos and Wisdom, and only-begotten Son.
“There is one body and one Spirit, just as also you were called in one hope of your calling; 5 one Lord, one faith, one baptism, 6 one God and Father of all who is over all and through all and in all.” Ephesians 4:4-5 NAS
“This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.” John 17:3 NAS
“yet for us there is but one God, the Father, from whom are all things and we exist for Him; and one Lord, Jesus Christ, by whom are all things, and we exist through Him.” 1 Corinthians 8:6 NAS
We see Athenagoras express his beliefs on the matter thus:
“But, since our doctrine acknowledges one God, the Maker of this universe, who is Himself uncreated (for that which is does not come to be, but that which is not) but has made all things by the Logos which is from Him, we are treated unreasonably in both respects, in that we are both defamed and persecuted.” A Plea For the Christians, Chapter IV.
“That we are not atheists, therefore, seeing that we acknowledge one God, uncreated, eternal, invisible, impassible, incomprehensible, illimitable, who is apprehended by the understanding only and the reason, who is encompassed by light, and beauty, and spirit, and power ineffable, by whom the universe has been created through His Logos, and set in order, and is kept in being—I have sufficiently demonstrated. [I say “His Logos”], for we acknowledge also a Son of God.” A Plea For the Christians, Chapter X.
From both these quotations we see Athenagoras identify that Christians believe in one God; and that this one God created all thing through His Logos, Who is His Son. Thus we see Him clearly equate the “one God” with the person of the Father, while the Logos stands in relation to the one God as His divine Son.
This same language is used by the Nicene Creed, when it says “We believe in one God, the Father Almighty, Maker of all things…”, afterwards going on to confess “one Lord Jesus Christ, the Son of God”, and one “Holy Spirit”.
For an extensive list of testimonies from the ante-nicene and nicene church fathers on this subject, see here: https://contramodalism.com/2017/03/08/i-believe-in-one-god-the-father-almighty/.