Heidegger's Bible Handbook: 3 John: Occasion and Argument of the Epistle
3. The occasion and argument of the Epistle.
The occasion for writing to Gaius was furnished by those that were praising the constancy of Gaius in John’s presence. On this occasion, Saint John rejoices over him, exhorts him to continue his beneficence toward the household of faith, marks Diotrephes, attests to Demetrius, and promises his own coming.