Christ is confessed [in Matthew 16] in two ways. First he is confessed according to His life. The disciples say to the Lord, “Some say You are John the Baptizer, others that You are Elijah. Some that You are Jeremiah or one of the prophets.” That is not yet a sure and rightly fashioned confession of Christ. It hangs only onto His outer appearance and the kind of life that Christ led. Many Jews confessed Him that way. So where there is only flesh and reason, people can grasp nothing more of Christ than that He is only a holy, good man who gives us a fine example which we should follow. Even if Christ were now walking here on earth, reason could still not confess anything more of Him than that. Now heaven is still closed to whoever takes Him as a holy man, as an example to be followed for a good life. He has not rightly grasped or confessed Christ. He only regards Him as a holy man, as also Elijah, Elisha, Jeremiah and other good saints had been. So take note of this rule. Where just plain reason appraises Christ He will only be regarded as a teacher and a holy man. But that is because it is not the Father, but one’s own reason, who is teaching his heart.
The other understanding of Christ is the one Peter has when he says here, “You are Christ, the Son of the living God.” He is saying, “You are a different kind of man. You are not like Elijah, nor John, nor Jeremiah nor anyone else who has come before. There is something greater about You. You are Christ, Son of the living God. You are beyond comparison to any saint, not to John nor to Elijah, nor to Jeremiah.” For if Christ is regarded only as a good man, then reason is in control tossing you to and fro, flitting from one to another, from Elijah to Jeremiah. But here He is singled out and regarded as something different than all the other saints. By this He is made sure. For if I have a dubious Christ then my conscience can never be settled. It can never find security.
So here a distinction is made between faith and works. Christ Himself makes it clear that He must not be grasped by works. You cannot receive Him by works for works follow after faith. First I must walk in His kindness, that He is mine and I am His. That can only happen by the working of the Word, where Peter says, “You are Christ, the Son of the living God.” Now blessed is he who has this confession of Christ. Reason cannot come this far. This is made known by Christ Himself when He answers Peter and says, “Blessed are you, Simon, son of John. Flesh and blood has not revealed this to you, but My Father in Heaven.” And He says further, “You are Peter (that is, a rock) and upon this rock I will build My church and the gates of hell shall not prevail against it.”
– Martin Luther, Church Postil for the Feast of SS Peter and Paul