Most ancient creeds, including the Nicene, begin by declaring faith in one God, the Father Almighty. This belief that God is one person, the Father of the Lord Jesus Christ (that is, Unitarianism) is given as the first article of the Christian faith in these ancient creeds and confessions. The identification of the one God with the person of the Father in particular is not only easily proved from the scriptures, but is also extensively witnessed to by the ante-nicene and nicene church fathers. A non-comprehensive list of quotes showing this is given below.
“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 4:4-6 NASB)
“This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.” (John 17:3 NASB)
“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 NASB)
“Now to Him who is able to keep you from stumbling, and to make you stand in the presence of His glory blameless with great joy, 25 to the only God our Savior, through Jesus Christ our Lord, be glory, majesty, dominion and authority, before all time and now and forever. Amen.” (Jude 1:24-25 NASB)
Ancient Patristic Witness:
Clement of Rome:
“Why are there strifes, and tumults, and divisions, and schisms, and wars among you? Have we not [all] one God and one Christ? Is there not one Spirit of grace poured out upon us?” 1 Clement, Chapter XLVI.
“We beseech Thee, Lord and Master, to be our help and succor. Save
those among us who are in tribulation; have mercy on the lowly; lift
up the fallen; show Thyself unto the needy; heal the ungodly; convert
the wanderers of Thy people; feed the hungry; release our prisoners;
raise up the weak; comfort the fainthearted. Let all the Gentiles
know that Thou art the God alone, and Jesus Christ is Thy Son, and
we are Thy people and the sheep of Thy pasture.” 1 Clement, Ch 59
“For concerning faith and repentance and genuine love and temperance
and sobriety and patience we have handled every argument, putting you
in remembrance, that ye ought to please Almighty God in righteousness
and truth and long suffering with holiness, laying aside malice and
pursuing concord in love and peace, being instant in gentleness; even
as our fathers, of whom we spake before, pleased Him, being lowly
minded toward their Father and God and Creator and towards all men.” 1 Clement, Ch 62
Ignatius of Antioch:
“On this account also they were persecuted, being inspired by His grace to fully convince the unbelieving that there is one God, who has manifested Himself by Jesus Christ His Son, who is His eternal Word, not proceeding forth from silence, and who in all things pleased Him that sent Him.” Epistle to the Magnesians (shorter version), Chapter VIII.