John was one of the TWELVE apostles that will sit on TWELVE thrones judging the TWELVE tribes of ISRAEL. John was not writing to the church which is His body... the body of Christ.
I'm not sure what Diotrephes has to do with anything, but he was an unbeliever that opposed John and the twelve.
3John 1:9-10 (AKJV/PCE)
(1:9) I wrote unto the church: but Diotrephes, who loveth to have the preeminence among them, receiveth us not. (1:10) Wherefore, if I come, I will remember his deeds which he doeth, prating against us with malicious words: and not content therewith, neither doth he himself receive the brethren, and forbiddeth them that would, and casteth [them] out of the church.
John spoke to the elect NATION, the nation of Israel.
The term "Christians" is a bit vague and does not accurately describe the divisions that God put in His plans. The "Christian church" for today is the BODY OF CHRIST and not the nation of Israel.
Yours is the confusion that comes when you do not rightly divide the word of truth.
Gal 2:9 (AKJV/PCE)
(2:9) And when James, Cephas, and John, who seemed to be pillars, perceived the grace that was given unto me, they gave to me and Barnabas the right hands of fellowship; that we [should go] unto the heathen, and they unto the circumcision.
That is a division that God created.