Howbeit we know this Man whence he is; that is, they knew his parentage, the place of his early life, the father, mother, brothers, and sisters (Matthew 13:55, Matthew 13:58). There was none of the mystery about him which they anticipated for their Messiah.
It is even intimated that it was known where he was born (John 7:41, John 7:42), and that the Christ would be born in Bethlehem, so that the mere fact of birthplace is not the difficulty that occurred to them.
A tradition had gathered, which was perhaps originated by Daniel 7:13 or Malachi 3:1, that he would make a sudden descent on the temple—a dazzling appearance at his Messianic enthronization, coming in the clouds of heaven, and that none would "declare his generation."
So, according to 'Sanh.,' 97, a, "three things are wholly unexpected—Messiah, a god-send, and a scorpion" (cf. 'Mid. on Song of Solomon 2:9'). Justin Martyr puts into the lips of Trypho, 'Dial.,' 8, "But Christ—if he has indeed been born, and exists anywhere—is unknown, and does not even know himself, and has no power until Elias come to anoint him and make him manifest to all."
So these Jerusalemites said, When the Christ cometh ( ἔρχηται makes his Christwise manifestation—is in act of coming), no one knoweth whence he is. This Messianic manifestation has been tardy and gradual, if it be one at all We know the home, the daily upbringing of Jesus—we know whence he is, or think we do; and so the whole affair clashes with a current expectation.
We know enough, too much, of this Jesus for it to be possible for him to fill up this portion of the Messianic programme. This may have been the outcome of the general criticism. Other defects, according to their idea, may have been urged.
The many-sidedness of the hope, the vagueness of the dream, as it shaped itself in current Jewish thought, suffered almost any amount of doubt as to the exact form of the approaching manifestation. That to which our Lord especially replied revealed the practical and ethical claim he advanced to their acceptance from himself of the word of the Lord.