Recently I had occasion to preach on John 16:12-15. Opening up a commentary or two I noticed reference to a variant reading in verse 13. Jesus, speaking of the Spirit's role, says, according to the NRSV,
'... he will not speak on his own, but will speak whatever he hears ...'
The Greek text for the second phrase reads
'but whatever he will hear (akousei) he will speak.'
An alternative reading of the second phrase is
'but whatever he hears (akouei) he will speak.'
Metzger in his A Textual Commentary on the Greek New Testament (2002) says that the variant is 'a dogmatic improvement, introduced to suggest the eternal relationship of the Holy Spirit with the Father' (p. 210). That is, a scribe working on Codex Sinaiticus, perhaps in the heady Trinitarian days following the Council of Nicea in 325 AD, thought that the Spirit speaking what he is yet to hear diminished the notion of the eternal relationship of the Spirit with the Father. A slight change with the omission of one letter, changing future tense to present tense, would restore the Spirit to his rightful place. (I myself think that the NRSV (also NIV) using the present tense is stylistic rather than a siding with Sinaiticus. Sinaiticus's sister codex, Vaticanus, incidentally, does not have this change).
There is a strong irony in a scribe seeking to improve John's Gospel in this way. John's Gospel itself is an attempt to improve ('improve') on the Synoptic Gospels in the sense that it consistently presents Jesus from the perspective of a deeper understanding of the relationship between the Father and the Son than disclosed by those gospels. It is John's Gospel which reveals to us that the Father and the Son are one. From that revelation flows the deduction that the Holy Spirit is one with Father and Son. The ancient anonymous scribe has made a well-intended Trinitarian error as he copied John's Gospel into the new codex and done so according to a Johannine way of thinking.
But many centuries later we might ask whether John himself was in error when he used the future tense in 16:13. However I do not think so. The use of the future tense through passages such as this, in Jesus' last testament to his disciples, consistently looks ahead to the ultimate revelation of the glory of God in Jesus Christ, to the event of his exaltation through the cross-and-resurrection or beyond it ('the things that are to come', 16:13c).
On that day the fullness of God's revelation through Jesus Christ is disclosed, the future tenses anticipate 'all the truth' available on that day, into which the Spirit 'will guide' the disciples (16:13a). John's time here is the earthly chronology in which the resurrection is yet to occur rather than the heavenly chronology in which that day is eternally present to God.
Thus John underscores the importance of the cross-and-resurrection as the single definitive event for Jesus. (If you want to push that, yes, John treats four things we often separate as one event of exaltation, cross-resurrection-ascension-pentecost). In this event, at least in a form of embryo which grows develops and flourishes, all the truth is revealed. The Spirit will speak to us what he will hear on that day.
The Sinaiticus scribe understood too little, not too much!