John Lightfoot on Matthew 16:13

[Whom do men say that I the Son of man am?] I. That phrase or title, the Son of man, which Christ very often gives himself, denotes not only his humanity, nor his humility (for see that passage, John 5:27, "He hath given him authority of executing judgment, because he is the Son of man"); but it bespeaks the 'seed promised to Adam, the second Adam': and it carried with it a silent confutation of a double ignorance and error among the Jews: 1. They knew not what to resolve upon concerning the original of the Messias; and how he should rise, whether he should be of the living, as we noted before, the manner of his rise being unknown to them; or whether of the dead. This phrase unties this knot and teaches openly, that he, being a seed promised to the first man, should arise and be born from the seed of the women. 2. They dreamed of the earthly victories of the Messias, and of nations to be subdued by him; but this title, The Son of man, recalls their minds to the first promise, where the victory of the promised seed is the bruising of the serpent's head, not the subduing of kingdoms by some warlike and earthly triumph.

II. When, therefore, the opinion of the Jews concerning the person of the Messias, what he should be, was uncertain and wavering, Christ asketh, not so much whether they acknowledged him the Messias, as acknowledging the Messias, what kind of person they conceived him to be. The apostles and the other disciples whom he had gathered, and were very many, acknowledged him the Messias: yea, those blind men, chapter 9:27, had confessed this also: therefore that question had been needless as to them, "Do they think me to be the Messias?" but that was needful, "What do they conceive of me, the Messias?" and to this the answer of Peter has regard, "Thou art Christ, the Son of the living God": as if he should say, "We knew well enough a good while ago that thou art the Messias: but as to the question, 'What kind of person thou art,' I say, 'Thou art the Son of the living God.'" See what we note at chapter 17:54.

Therefore the word whom asks not so much concerning the person, as concerning the quality of the person. In which sense also is the word who, in those words, 1 Samuel 17:55, not "The son of whom," but the son "of what kind of man," is this youth?