The elder says Diotrephes talks against them with “wicked words.” He also refuses to welcome the brothers and stops others from doing so, putting them out of the church (3 John 1:10).
In 3 John 9–10 the writer explains why cooperation with traveling believers has broken down in a local group. He had previously written to the assembly, but a man named Diotrephes refuses to accept the writer’s message because he wants the top place. These verses sit in the middle of a short letter addressing practical support and welcome for traveling believers. Earlier, the writer praises Gaius for faithful help and says that such support makes people partners in the work (verses 5–8).
The passage describes a breakdown in cooperation between the letter’s writer (“I”) and a local assembly. The writer says he previously wrote to the assembly, but Diotrephes “doesn’t accept what we say” (an explicit claim).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with 3 John 1:10, then read the surrounding section so the answer stays connected to the passage's flow.
10Therefore, if I come, I will call attention to his deeds which he does, unjustly accusing us with wicked words. Not content with this, neither does he himself receive the brothers, and those who would, he forbids and throws out of the assembly.