A solemn warning is given, which forever discriminates the ministry of man from the eternal ministry of the Logos. He (John, and with him all the prophetic, Levitical, ascetic teachers in all ages) was not the Light, but [he was or came] that he might bear witness of the Light.
The ἵνα depends upon some unexpressed verbal thought; for even in the passages where it stands alone (John 9:3; John 13:18; John 14:31; John 15:25) the reference is not obscure to some pre-existing or involved verb.
The distinction here drawn between John and the Light is thought by some expositors to point to the condition of the Ephesian Church, in the neighbourhood of which there still lingered some who placed John in even a higher position than that accorded to Jesus (Acts 19:3, Acts 19:4); but the teaching of the evangelist is far more comprehensive than this.
The Light of men has higher source and wider range of operation than that of any prophetic man. All that he, that any seer whatsoever can do, is to bear witness to it. The prophets, from Moses to John, derived all their power, their sanction, and the corroboration of their message, from the Logos light shining through conscience and blazing through providential events and burning up the stubble of human action with unquenchable fire.
The prophets are not the light of God; they are sent to bear witness to it.