The people answered him, We have heard out of the law that Christ abideth for ever: and how sayest thou, The Son of man must be lifted up? who is this Son of man? We have heard out of the law - That is, out of the sacred writings. The words here are quoted from Psalms 110:4; but the Jews called every part of the sacred writings by the name, The Law, in opposition to the words or sayings of the scribes. See on John 10:34 (note).
That Christ abideth for ever - There was no part of the law nor of the Scripture that said the Messiah should not die; but there are several passages that say as expressly as they can that Christ must die, and die for the sin of the world too. See especially Isaiah 53:1, etc.; Daniel 9:24, Daniel 9:27. But as there were several passages that spoke of the perpetuity of his reign, as Isaiah 9:7; Ezekiel 37:25; Daniel 7:14, they probably confounded the one with the other, and thus drew the conclusion, The Messiah cannot die; for the Scripture hath said, his throne, kingdom, and reign shall be eternal. The prophets, as well as the evangelists and apostles, speak sometimes of the Divine, sometimes of the human nature of Christ: when they speak of the former, they show forth its glory, excellence, omnipotence, omniscience, and eternity; when they speak of the latter, they show forth its humiliations, afflictions, sufferings, and death. And those who do not make the proper distinction between the two natures of Christ, the human and the Divine, will ever make blunders as well as the Jews. It is only on the ground of two natures in Christ that the Scriptures which speak of him, either in the Old or New Testament, can be possibly understood. No position in the Gospel is plainer than this, God was manifest in the flesh.