Jamieson, Fausset and Brown on John 12:34

34. We have heard out of the law--the scriptures of the Old Testament (referring to such places as Psa 89:28, 29; 110:4; Da 2:44; 7:13, 14).

that Christ--the Christ "endureth for ever."

and how sayest thou, The Son of Man must be lifted up, &c.--How can that consist with this "uplifting?" They saw very well both that He was holding Himself up as the Christ and a Christ to die a violent death; and as that ran counter to all their ideas of the Messianic prophecies, they were glad to get this seeming advantage to justify their unyielding attitude.