Isaiah said these things because he saw Jesus’ glory and spoke about Him.
41. These things spoke Jesus. Lest readers should think that this prediction was inappropriately quoted, John expressly states, that the prophet was not sent as a teacher to a single age, but, on the contrary, that the glory of Christ was exhibited to him, that he might be a witness of those things which should take place under his reign.
We have here the honour done to our Lord Jesus by the Old Testament prophets, who foretold and lamented the infidelity of the many that believed not on him. It was indeed a dishonour and grief to Christ that his doctrine met with so little acceptance and so much opposition; but this takes off the wonder and reproach, makes the offence of it to cease...
Commenting on John 12:37-41
Nevertheless among the chief rulers also,.... These were the members of the Jewish sanhedrim, as Nicodemus, Joseph of Arimathea, and others: many believed on him; that he was the Messiah, though they did not believe in him in a spiritual and saving manner, as their Redeemer and Saviour, only in their minds, being convicted by his miracles, gave an assent unto him, as the promised Messiah.