A.T. Robertson Commentary John 12:34

"The multitude therefore answered him, We have heard out of the law that the Christ abideth for ever: and how sayest thou, The Son of man must be lifted up? who is this Son of man?" — John 12:34 (ASV)

Out of the law (εκ του νομου). That is, "out of the Scriptures" (10:34; 15:25).

The Christ abideth forever (ο Χριστος μενε εις τον αιωνα). Timeless present active indicative of μενω, to abide, remain. Perhaps from Ps 89:4; 110:4; Isaiah 9:7; Ezekiel 37:25; Daniel 7:14.

How sayest thou? (πως λεγεις συ; ). In opposition to the law (Scripture).

The Son of man (τον υιον του ανθρωπου). Accusative case of general reference with the infinitive υψωθηνα (first aorist passive of υψοω and taken in the sense of death by the cross as Jesus used it in verse 32). Clearly the crowd understand Jesus to be "the Son of man" and take the phrase to be equivalent to "the Christ." This is the obvious way to understand the two terms in their reply, and not, as Bernard suggests, that they saw no connexion between "the Christ" (the Messiah) and "the Son of man." The use of "this" (ουτος) in the question that follows is in contrast to verse 32. The Messiah (the Son of man) abides forever and is not to be crucified as you say he "must" (δε) be.