John Calvin Commentary John 6:69

"And we have believed and know that thou art the Holy One of God." — John 6:69 (ASV)

And we have believed and known. The verbs are in the past tense but can be changed to the present tense—we believe and know—though it makes little difference to the meaning. In these words, Peter gives a brief summary of faith. However, the confession appears to have nothing to do with the matter at hand, as the question had been raised about eating the flesh of Christ.

I reply, although the twelve did not immediately understand all that Christ had taught, it is enough that, according to the capacity of their faith, they acknowledge Him as the Author of salvation and submit themselves to Him in all things. The word believe is put first because the obedience of faith is the beginning of correct understanding, or rather, because faith itself is truly the eye of the understanding.

But immediately afterward, knowledge is added, which distinguishes faith from erroneous and false opinions. For Muslims, Jews, and Roman Catholics believe, but they neither know nor understand anything. Knowledge is connected with faith because we are certain and fully convinced of the truth of God—not in the same way that human knowledge is acquired, but when the Spirit seals it on our hearts.