John 1:7: οὗτος ἦλθεν εἰς μαρτυρίαν, ἵνα μαρτυρήσῃ περὶ τοῦ φωτός, ἵνα πάντες πιστεύσωσιν δι' αὐτοῦ.
Everytime I have read this, without really thinking about it, I assumed that the αὐτοῦ refers to the light=Jesus.
"...so that everyone might believe through it/him (i.e. the light, that is Jesus.")
But I note that Nonnus' paraphrase has
Nonnus 1:7b: ἵνα πάντες ἑνὸς κήρυκος ἰωῇ ὀρθὴν πίστιν ἔχοιεν...
...so that all might through the voice of one preacher have faith...
The κήρυξ presumably being John the Baptist, not Jesus. Since this is of course a loose paraphrase, this does not mean that Nonnus took the αὐτοῦ to mean John, but I think that is the implication. I know that if this were meant, John would have more properly written ἐαυτοῦ, but I also know that Johannine grammar can be rather loose.
Every translation I looked at can be read either way, so I am not even sure how this verse is contrued by the translators. Todays Greek Version is also ambiguous, though I think it implies John is meant
TGV John 1:7b: ...ώστε με τα λόγια του να πιστέψουν όλοι.
"so that through his words all might believe."
Presumably the words of John, not Jesus.
It probably does not matter. Faith is normally construed as IN (εἰς or ἐν) Jesus and THROUGH (διά) some remoter agency like a preacher, John being the first and one of the best.
I'm now curious though, whether people think Jesus or John is the referent of αὐτοῦ.