The Book of 3rd John
"Diotrephes, who loves to be first among them, does not accept what we say."

Bible Study and Commentary
James Deering,

conjunctions & prepositions underlined, words not in original text in italics)  (alternate text in purple)

3 John :9
I wrote something to the church;
    but Diotrephes,
        who loves to be first among them,
        does not accept what we say.

3 John :10
For this reason,
                if I come,
                I will call attention to his deeds which he does,
                    unjustly accusing us with wicked words; and
                    not satisfied with this,
                    neither does he himself receive the brethren, and
                    he forbids those who desire to do so, and
                    puts them out of the church.

The best implication would be that John is writing to Gaius instead of the church, because he knows that, thanks to the influence of Diotrephes, a letter to the church may not even reach their ears - if Diotrephes see it first. A suggestion has been made that John is here making a reference to the problem seen in 2 John – that, the elect lady (either a person or the church as a whole) has been unresponsive to his earlier letter. John had warned her / them that they should be more discriminating in their choices of fellowship, that false teachers may creep into the fold. John had now to write to one of her children (Gaius). It may be that the first letter failed because Diotrephes, a dominant personality in that church, forbad his brethren to comply with the Elder’s request. The language suggests that Diotrephes was, "a self-promoted demagogue, rather than a constitutional elder"(F.F. Bruce, noted above). It is conceivable, of course, that even a constitutional leader might have been regarded by the Elder (John) as no better than a trumped-up dictator if he behaved in the way described here. He does not receive us, says John; that is to say, he neither recognized John’s authority nor admitted his messengers to the church.

John starts off this sentence with "for this reason." So we look back (similar to the earlier therefore) and see that the reason for this next admonition is based in Diotrephes refusal to allow certified traveling missionaries, preachers, and teachers into the church, or to help support them and their work, or perhaps even into the city.

John is no private individual. He is after all, John, The one whom Jesus loved, the Apostle, the son of Zebedee, the writer of the Gospel of John, and one who is capable of speaking authoratatively to Diotrephes and to the local church.

John states that should he travel to that church, because of these things, then Diotrephes will have to answer for his behavior.

Diotrephes will be addressed in the following areas:

his deeds