"'For I have come down from heaven, not to do My own will, but the will of Him who sent Me. This is the will of the Father who sent Me, that of all he has given Me I should lose nothing, but should raise it up at the last day.' The Jews then complained about Him, because He said, 'I am the bread which came down from heaven.' And they said, 'Is not this Jesus, the son of Joseph, whose father and mother we know? How is it then that He says, "I have come down from heaven?"' Jesus therefore answered and said to them, 'Do not murmur among yourselves.' (John 6:38-43).
Jesus declared to the Jews that He came down from heaven. They, of course, would object to this statement. Jesus' purpose in coming down from heaven was to fulfill the Father's will, which He explains in 6:39. The Father desires that the Son lose nothing of that which He gave Him and that He "should raise it up at the last day." Note the close parallel between this verse and 6:40. From it we learn that the Father has given to Jesus those who see the Son and believe in Him. Such individuals will not be lost but will have everlasting life after being resurrected on the last day. Thus, in essence, God's will for all mankind is that they believe in Jesus so that He may give them eternal life. Jesus made it clear as to where He was from and what He was here to do. The choice was theirs (and ours) as to whether or not to believe in Him.
Regarding the mentioning of the Father's will, the people should have asked themselves: "Would it even be possible for Jesus to work signs and miraculous wonders if His teaching was contrary to the Father's will?" They should have believed Him for the miracles they had already witnessed (cf. John 20:30,31) instead of acting as if He hadn't shown any works to substantiate His claims.
John 6:41 records the Jews complaining about Jesus' claims. Remember that these Jews had begun a discussion with the Lord as to whether or not He was greater than Moses. They didn't believe He was, so it is not surprising that they rejected His claim of being "bread which came down from heaven." For Jesus to assert that He came down from heaven is to claim participation in a glory that would make Him clearly superior to all other prophets. They were not willing to accept this truth.
The primary reason given for their rejection of His claim was that they were familiar with His parents, or so they thought. They believe His claim of coming down from heaven is preposterous for they knew Mary and Joseph!
Jesus began His response to their grumbling - "Do not murmur among yourselves" (John 6:43). They did not know that Jesus didn't have a true, physical father, but Jesus doesn't bother to give them that information. Certainly He knew their hearts and revealing the truth about His being born of a virgin wouldn't have persuaded them anyway. He merely tells them to quit complaining as He prepares to do more teaching.