I found this quote of interest, from an introduction to 2 John from a Catholic edition of the Bible, the New American Bible.
They certainly didn’t have us in mind when this was written, but I certainly find it applicable. Shortly, I’ll have a specific situation to speak on.
The translation of verse 9 is suggestive also: “Anyone who is so ‘progressive’ as not to remain in the teaching of the Christ does not have God; whoever remains in the teaching has the Father and the Son.”