This ringing affirmation of faith came from Peter as spokesman, but undoubtedly was shared by all the disciples, since Jesus had asked the question "Whom say ye that I am?" of them all. Actually, they had probably all been disciples of John the Baptist, who had directed them to Jesus, and so had heard John's testimony concerning Christ's identity. John had said that Jesus was indeed "the only begotten Son, which is in the bosom of the Father" (John 1:18).
Yet, as they had been following Him, they had heard Him speak of Himself far more often as "the Son of man." Over thirty times, in the gospel of Matthew alone, He identified Himself as Son of man, not once as the Son of God. Nevertheless, He accepted Peter's statement as absolutely true, saying that the Father had so revealed it.
In fact it is essential that one must believe it to be saved. Jesus did say: "But he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God" (John 3:18).
Yet He seems to want us to know Him especially as the Son of man, perhaps so that we will never forget that He, though God, is also man just like us. And as man, He was "in all points |tested| like as we are, yet without sin" so He can "be touched with the feeling of our infirmities," and we now can "come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need" (Hebrews 4:15-16).
John was enabled to see Christ once again, long after His return to heaven. Although He was now in His resurrection body, John still saw Him as "one like unto the Son of man" (Revelation 1:13). Although He is indeed the Son of the living God, He is also our "man in the glory"! HMM http://www.icr.org/article/5971/