I have written to the church about this: This could refer to 1 John or to a letter that is now lost.
Diotrephes was proud and thought himself important. He was motivated by selfish ambition (see 1 Timothy 3:6).
will not accept our instruction: This disagreement was a major issue that John addressed in 1 John (see, for example, 1 John 2:19; 4:5–6). Diotrephes rejected the teachers sent by John and removed any church members who accepted them. He wanted to lead the local church without answering to any outside authority.