What is the theology of 3 John? What is the key verse of the book of 3 John?

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The purpose of the letter is to encourage and strengthen Gaius, and to warn him against Diotrephes, who refuses to cooperate with the author of the letter. Early church literature contains no mention of the epistle, with the first reference to it appearing in the middle of the third century AD.
What lessons can we learn from the book of 3 John?
3 John Lesson
In his third epistle, John calls him out for:
The examples of Gaius and Diotrephes show us that John paid attention to the types of people that he associated with.
So should we!
It matters to God who we choose to associate with.
Why?
Because the people that we associate with influence how we b