They were asking, “Is this not Jesus, the son of Joseph, whose father and mother we know? How then can He say, ‘I have come down from heaven’?”
Whether this conference was with the Capernaites, in whose synagogue Christ now was, or with those who came from the other side of the sea, is not certain nor material; however, it is an instance of Christ's condescension that he gave them leave to ask him questions, and did not resent the interruption as an affront, no, not from his common hearers, though not his immediate followers.
Commenting on John 6:28-59
Jesus therefore answered and said unto them,.... Either overhearing what they said, or knowing, as God, their secret murmurs, and private cavils among themselves, thus addressed them, murmur not among yourselves: meaning neither about his descent from heaven, nor about coming to him, and believing in him; for it follows,