2 But how can it be any longer one faith, if it does not steadfastly and sincerely confess one Lord and one God the Father: and how can the faith which is not one faith confess one Lord and one God the Father? Further, how can the faith be one, when its preachers are so at variance? One comes teaching that the Lord Jesus Christ, being in the weakness of our nature, groaned with anguish when the nails pierced His hands, that He lost the virtue of His own power and nature, and shrank shuddering from the death which threatened Him. Another even denies the cardinal doctrine of the Generation and pronounces Him a creature. Another will call Him, but not think Him, God on the ground that religion allows us to speak of more Gods than One, but He, Whom we recognise as God, must be conscious of sharing the divine nature. Again, how can Christ the Lord be one, when some say that as God He feels no pain, others make Him weak and fearful: to some He is God in name, to others God in nature: to some the Son by Generation, to others the Son by appellation? And if this is so, how can God the Father be one in the faith, when to some He is Father by His authority, to others Father by generation, in the sense that God is Father of the universe?
And yet, who will deny that whatever is not the one faith, is not faith at all? For in the one faith there is one Lord Christ, and God the Father is one. But the one Lord Jesus Christ is not one in the truth of the confession, as well as in name, unless He is Son, unless He is God, unless He is unchangeable, unless His Sonship and His Godhead have been eternally present in Him. He who preaches Christ other than He is, that is, other than Son and God, preaches another Christ. Nor is he in the one faith of the one baptism, for in the teaching of the Apostle the one faith is the faith of that one baptism, in which the one Lord is Christ, the Son of God Who is also God.
Source: On the Trinity (New Advent)