3 John is the most personal of the three short letters. The elder writes to a man he loves in truth — Gaius — and holds him up as an example of walking in truth and supporting those who go out for the sake of the Name.
The letter also names the opposite spirit: Diotrephes, who loves to be first, refuses the brothers, and throws out of the assembly those who would receive them.
Read 3 John as a snapshot of real life in the assemblies during the closing decades of the old covenant age. Truth is already present. Love is already being practised by some. Opposition is already active. The call is clear: continue walking in the truth and become fellow workers with those who go out for the Name.