The most evident threat in all 1-3 John is Docetism.
See especially 1 John 4:2 and 2 John 7: The Apostle John warns that they are antichrists.
John wrote this letter to a local congregation, warning against the same false teaching as in 1 John: Docetism.
John warns of the danger of being hospitable to those false teachers (docetists).
John teaches that true fellowship is only possible under sound doctrine.
John warns of the double danger of the church: (1) false teaching (docetism) and (2) division.
So what we need is (1) discernment and (2) love.
Diotrephes – loves to be first
Demetrius – well spoken of by everyone