A.T. Robertson Commentary John 1:7

"The same came for witness, that he might bear witness of the light, that all might believe through him." — John 1:7 (ASV)

For witness (εις μαρτυριαν). Old word from μαρτυρεω (from μαρτυς), 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 (ινα μαρτυρηση). Final clause with ινα and aorist active subjunctive of μαρτυρεω to make clearer εις μαρτυριαν.

Of the light (περ του φωτος). "Concerning the light." The light was shining and men with blinded eyes were not seeing the light (John 1:26), blinded by the god of this world still (2 Corinthians 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 (ινα πιστευσωσιν). Final clause with ινα and first aorist active subjunctive of πιστευω, 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 πιστις, so common in Paul, John uses only in 1 John 5:4 and four times in the Apocalypse where πιστευω does not occur at all. Here it is used absolutely as in Joh 1:50, etc.

Through him (δι' αυτου). 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.