John Calvin Commentary John 12:41

"These things said Isaiah, because he saw his glory; and he spake of him." — John 12:41 (ASV)

These things spoke Jesus. So that readers would not think that this prediction was inappropriately quoted, John explicitly states that the prophet was not sent as a teacher to a single age, but that, on the contrary, the glory of Christ was revealed to him so that he might be a witness to those things that would take place under His reign. Now the Evangelist takes for granted that Isaiah saw the glory of Christ; and therefore he infers that Isaiah accommodates his instruction to the future state of Christ’s kingdom.