12 “The people answered Him, We have heard out of the law that Christ abides for ever: and how sayest Thou, The Son of man must be lifted up? And who is this Son of man?” It had stuck to their memory that the Lord was constantly calling Himself the Son of man. For, in the passage before us, He does not say, If the Son of man be lifted up from the earth; but had called Himself so before, in the lesson which was read and expounded yesterday, when those Gentiles were announced who desired to see Him: “The hour has come that the Son of man should be glorified”.
Retaining this, therefore, in their minds, and understanding what He now said, “When I am lifted up from the earth,” of the death of the cross, they inquired of Him, and said, “We have heard out of the law that Christ abides for ever; and how sayest Thou, The Son of man must be lifted up? Who is this Son of man?” For if it is Christ, He, they say, abides for ever; and if He abides for ever, how shall He be lifted up from the earth, that is, how shall He die through the suffering of the cross? For they understood Him to have spoken of what they themselves were meditating to do. And so He did not dissipate for them the obscurity of such words by imparting wisdom, but by stimulating their conscience.
Source: Tractates on the Gospel of John (New Advent)