ISB ~ 3 John 1 v12

I testify to him:Demetrius appears to be the bearer of 3 John. If so, the letter also serves as a letter of recommendation for him. Many such letters were written in the ancient Church to prepare the way for traveling missionaries going from place to place (Acts 18:27; Rom 16:1-2; 2 Cor 3:1). John hopes Gaius will welcome godly men like Demetrius (3 Jn 8), whom Diotrephes turns away (10).