In addition to being ambitious, proud, disrespectful of apostolic authority, rebellious, and inhospitable, the author of the letter says that Diotrephes tried to hinder those desiring to show hospitality to the brothers and to expel these from the congregation. Not even the location of Diotrephes' church can be determined from the letter. It is debatable whether the antipathy expressed in 3 John is based on "a theological dispute, a clash of competing ecclesiastical authorities, a disagreement about financial responsibilities for the mission, or personal dislike".
Biblical passage 
The following is the passage and notes from the New English Translation.
1:9 I wrote something to the church, but Diotrephes, who loves to be first among them, does not acknowledge us. 1:10 Therefore, if I come, I will call attention to the deeds he is doing – the bringing of unjustified charges against us with evil words! And not being content with that, he not only refuses to welcome the brothers himself, but hinders the people who want to do so and throws them out of the church! 1:11 Dear friend, do not imitate what is bad but what is good. The one who does good is of God; the one who does what is bad has not seen God.
Subsequent References 
In 1588, the Elizabethan Puritan John Udall wrote a dialogue with a haughty bishop named Diotrephes. Writing anonymously, Udall claimed that his godly and witty protagonist, Paul, was merely cautioning the English bishops to be wary of false counselors, particularly the Catholics from whom they had inherited the structure of English ecclesiology. Although the dialogue's actual title is The state of the Church of Englande, laide open in a conference betweene Diotrephes a byshop, Tertullus a papist, Demetrius an vsurer, Pandocheus an inne-keeper, and Paule a preacher of the worde of God, it is commonly referred to by scholars as Diotrephes.
- Raymond E. Brown, An Introduction to the New Testament (New York: Doubleday, 1997), p. 403
- Margaret M. Mitchell, "'Diotrephes Does Not Receive Us;: The Lexicographical and Social Context of 3 John 9-10," Journal of Biblical Literature 117.2 (1998:299-320), with bibliography of the conflict on each possible arena of the conflict
- Harnack, "Über den dritten Johannesbrief" (series Texte und Untersuchungen zur Geschichte der altchristlichen Literatur) 15.3 (Leipzig 1897:3-27).
- sn The church mentioned here, which the author says he may visit (3 John 10) is not the same as the one mentioned in 3 John 6, to which the author apparently belongs (or of which he is in charge). But what is the relationship of this church in v. 9 to Gaius, to whom the letter is addressed? It is sometimes suggested that Gaius belongs to this church, but that seems unlikely, because the author uses a third-person pronoun to refer to the other members of the church (among them). If Gaius were one of these it would have been much more natural to use a second-person pronoun: “Diotrephes, who loves to be first among you.” Thus it seems probable that Gaius belongs to (or is in charge of) one local church while Diotrephes is in another, a church known to Gaius but to which he does not belong.
- sn Diotrephes appears to be an influential person (perhaps the leader) in a local church known to Gaius, but to which Gaius himself does not belong. The description of Diotrephes as one who loves to be first suggests he is arrogant, and his behavior displays this: He refuses to acknowledge the written communication mentioned by the author at the beginning of v. 9 (and thus did not recognize the author’s apostolic authority), and furthermore (v. 10) refuses to show any hospitality to the traveling missionaries (welcome the brothers) already mentioned by the author. It has been suggested that the description “loves to be first” only indicates that Diotrephes sought prominence or position in this church, and had not yet attained any real authority. But his actions here suggest otherwise: He is able to refuse or ignore the author’s previous written instructions (v. 9), and he is able to have other people put out of the church for showing hospitality to the traveling missionaries (v. 10).
- tn Since the verb ἐπιδέχομαι (epidechomai) can mean “receive into one’s presence” (BDAG 370 s.v. 1; it is used with this meaning in the next verse) it has been suggested that the author himself attempted a previous visit to Diotrephes’ church but was turned away. There is nothing in the context to suggest an unsuccessful prior visit by the author, however; in 3 John 9 he explicitly indicates a prior written communication which Diotrephes apparently ignored or suppressed. The verb ἐπιδέχομαι can also mean “accept” in the sense of “acknowledge someone’s authority” (BDAG 370 s.v. 2) and such a meaning better fits the context here: Diotrephes has not accepted but instead rejected the authority of the author to intervene in the situation of the traveling missionaries (perhaps because Diotrephes believed the author had no local jurisdiction in the matter).
- tn The third-class condition (ἐὰν ἔλθω, ean elthō) seems to be used by the author to indicate real uncertainty on his part as to whether he will visit Diotrephes’ church or not.
- sn Because Diotrephes did not recognize the authority of the author, the author will expose his behavior for what it is (call attention to the deeds he is doing) if he comes for a visit. These are the charges the author will make against Diotrephes before the church: (1) Diotrephes is engaged in spreading unjustified charges against the author with evil words; (2) Diotrephes refuses to welcome the brothers (the traveling missionaries) himself; (3) Diotrephes hinders the others in the church who wish to help the missionaries; and (4) Diotrephes expels from the church (throws them out) people who aid the missionaries. (Diotrephes himself may not have had supreme authority in the local church to expel these people, but may have been responsible for instigating collective action against them.)
- sn The exhortation do not imitate what is bad but what is good is clearly a reference to Diotrephes’ evil behavior. The author exhorts Gaius (whom he wishes to continue assisting the missionaries) not to follow the negative example of Diotrephes, but to do what is right. Implicitly there may be a contrast between the bad behavior of Diotrephes and the good reputation of Demetrius (mentioned in the following verse); but it seems more likely that Demetrius is himself one of the traveling missionaries (perhaps their leader), rather than the leader of a local congregation who, unlike Diotrephes, has supported the missionaries himself.
- sn The statement The one who does what is bad has not seen God is asyndetic; its abrupt introduction adds emphasis. The statement reiterates the common Johannine theme of behavior as an indication of genuine faith, found in 1 John in 3:6, 10; 4:7, 20; and in the Gospel of John in 3:17-21. By implication, the genuineness of Diotrephes’ faith is called into question, because he has obviously done what is bad (v. 11b; cf. vv. 9-10). In John’s terminology it is clear that the phrase has not seen God is equivalent to “is not a genuine Christian” (see John 3:17-21 and 1 John 3:6, 10; 4:7, 20).
- sn British Library's English Short Title Catalogue, STC (2nd ed.), 24505