John Chrysostom
Catena Aurea · Johannesevangelium 1
There was a man sent from God, whose name was John. The same came for a witness, to bear witness of the Light, that all men through him might believe. He was not that Light, but was sent to bear witness of that Light.
(Hom. vi. in Joh. c. 1) Forasmuch however as with us, the one who witnesses, is commonly a more important, a more trustworthy person, than the one to whom he bears witness, to do away with any such notion in the present case the Evangelist proceeds; He was not that Light, but was sent to bear witness of that Light. If this were not his intention, in repeating the words, to bear witness of the Light, the addition would be superfluous, and rather a verbal repetition, than the explanation of a truth.