If any mere confession whatsoever of Christ were the perfection of faith, it would have been said, Dost thou believe in Christ? But inasmuch as all heretics would have had this name in their mouths, confessing Christ, and yet denying the Son, that which is true of Christ alone, is required of our faith, viz. that we should believe in the Son of God. But what availeth it to believe on the Son of God as being a creature, when we are required to have faith in Christ, not as a creature of God, but as the Son of God.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: vi. de Trin. circa fin.