John Calvin on Matthew 16:15

15. But who do you say that I am? Here Christ distinguishes his disciples from the rest of the crowd, to make it more fully evident that, whatever differences may exist among others, we at least ought not to be led aside from the unity of faith. They who shall honestly submit to Christ, and shall not attempt to mix with the Gospel any inventions of their own brain, will never want the true light. But here the greatest vigilance is necessary, that, though the whole world may be carried away by its own inventions, believers may continually adhere to Christ. As Satan could not rob the Jews of the conviction which they derived from the Law and the Prophets, that Christ would come, he changed him into various shapes, and, as it were, cut him in pieces. His next scheme was, to bring forward many pretended Christs, that they might lose sight of the true Redeemer. By similar contrivances, he continued ever afterwards either to tear Christ in pieces, or to exhibit him under a false character. Among the confused and discordant voices of the world, let this voice of Christ perpetually sound in our ears, which calls us away from unsettled and wavering men, that we may not follow the multitude, and that our faith may not be tossed about amongst the billows of contending opinions.