{For witness} (eis marturian). Old word from marture(935c)
(from martus), both more common in John's writings than the
rest of the N.T. This the purpose of the Baptist's ministry.
{That he might bear witness} (hina martur(8873)(8869)). Final clause
with hina and aorist active subjunctive of marture(935c) to make
clearer eis marturian. {Of the light} (peri tou ph(9374)os).
"Concerning the light." The light was shining and men with
blinded eyes were not seeing the light ( Joh 1:26 ), blinded by
the god of this world still ( 2Co 4:4 ). John had his own eyes
opened so that he saw and told what he saw. That is the mission
of every preacher of Christ. But he must first have his own eyes
opened. {That all might believe} (hina pisteus(9373)in). Final
clause with hina and first aorist active subjunctive of
pisteu(935c), ingressive aorist "come to believe." This is one of
John's great words (about 100 times), "with nine times the
frequency with which it is used by the Synoptists" (Bernard). And
yet pistis, so common in Paul, John uses only in  1Jo 5:4  and
four times in the Apocalypse where pisteu(935c) does not occur at
all. Here it is used absolutely as in  Joh 1:50 , etc. {Through
him} (di' autou). As the intermediate agent in winning men to
believe in Christ (the Logos) as the Light and the Life of men.
This is likewise the purpose of the author of this book
( 21:31 ). The preacher is merely the herald to point men to
Christ.