A.T. Robertson Commentary John 1:8

"He was not the light, but [came] that he might bear witness of the light." — John 1:8 (ASV)

He (εκεινος). "That one," i.e. John. He was a light (John 5:35) as all believers are (Matthew 5:14), but not "the light" (το φως).

But came (αλλ'). No verb in the Greek, to be supplied by repeating ηλθεν of verse 7. See similar ellipses in 9:3; 13:18; 15:25. In Johannine fashion we have the final ινα clause of verse 7 repeated.