3 Let us then ask Christ Himself, and learn from His own mouth, what is the chief means of our salvation. Who shall enter, O Christ, into Your kingdom? He that does the will of my Father in heaven, is His reply; not, "He that confesses that I was born." And again, He says to His disciples, "Go, teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost, teaching them to observe all things which I have commanded you." It is not, "teaching them that I was born," but, "to observe my commandments."
Again, "You are my friends if you do what I command you;" not, "if you believe that I was born." Again, "If you keep my commandments, you shall abide in my love," and in many other places. Also in the sermon on the mount, when He taught, "Blessed are the poor, blessed are the meek, blessed are the peacemakers, blessed are the pure in heart, blessed are they that mourn, blessed are they that hunger, blessed are they that are persecuted for righteousness' sake," He nowhere says, "Blessed are they that confess that I was born."
And in the separation of the sheep from the goats in the judgment, He says that He will say to them on the right hand, "I was hungry, and you gave me meat; I was thirsty, and you gave me drink," and so on; therefore "inherit the kingdom." Not, "Because ye believe that I was born, inherit the kingdom." Again, to the rich man seeking for eternal life, He says, "Go, sell all that you have, and follow me;" not, "Believe that I was born, that you may have eternal life." You see, the kingdom, life, happiness, are everywhere promised to the part I have chosen of what you call the two parts of faith, and nowhere to your part.
Show, if you can, a place where it is written that whoso confesses that Christ was born of a woman is blessed, or shall inherit the kingdom, or have eternal life. Even supposing, then, that there are two parts of faith, your part has no blessing. But what if we prove that your part is not a part of faith at all? It will follow that you are foolish, which indeed will be proved beyond a doubt. At present, it is enough to have shown that our part is crowned with the beatitudes. Besides, we have also a beatitude for a confession in words: for we confess that Jesus Christ is the Son of the living God; and Jesus declares with His own lips that this confession has a benediction, when He says to Peter, "Blessed are you, Simon Barjona; for flesh and blood has not revealed this unto you, but my Father which is in heaven." So that we have not one, but both these parts of faith, and in both alike are we pronounced blessed by Christ; for in one we reduce faith to practice, while in the other our confession is unmixed with blasphemy.
Source: Reply to Faustus the Manichaean (New Advent)