v1-2: The prayerful introduction; John loved and respected Gaius. Some have suggested he became leader of the church in Pergamum.
v3-8: The commendation of Gaius. He was walking in the truth, bringing joy to John, see 2 John 4. This was an evidence of spiritual life, and John holds him up as an example to others. John is writing to one he had won to Christ, and discipled. Now Gaius served God by showing unselfish concern for others, in showing hospitality to the brethren, and to strangers. Such hospitality is part of the way in which we work together for the truth. As in John's first letter, there is commendation for obedience and love for the brethren.
v9-10: The bad example of Diotrephes. John had previously written to the church, but Diotrephes rejected the apostle and his letter, and so John felt compelled to write to Gaius.
Diotrephes maintained some authority in the church, although it is clear that he had misused it.
Diotrephes loved to be first, to have the pre-eminence, seeking attention and praise, and followers for himself. Contrast Col 1.18, for Christ is to be pre-eminent in the church.
Within the church, we may see a person who is unwilling to be submissive; he will be involved, but only on his terms, as long as things are done his way. In contrast, there is the gracious and submissive attitude, willing to prefer others, putting their needs first, not grumbling when others have their way.
Diotrephes had "nothing to do with us," attacking God's appointed leaders. He knew who John was, but was unwilling to respect his authority. John was planning to expose him, calling attention to what he was doing.
It is good to have a teachable spirit, willing to learn from others.
Diotrephes gossiped maliciously about John and his companions, intending evil, designed to cause hurt. Even if a man speaks the truth, but intends harm, then he is speaking maliciously. It reveals a lack of love, and an unholy attitude.
It is better to minister grace (Eph 4.29), seeking to build others up.
Diotrephes did not welcome the brethren, despising true Christians. He had his own small group of friends, who were doubtless powerful within the church, and had little time for others. He had even put some out of the church. When others wanted to receive the brethren, he forbade them.
John's description of Diotrephes seems to match Jesus' words in Luke 9.46-56; Jesus rebuked those who would not welcome others, and those who criticised other disciples, and those who wanted to judge others.
v11: How should we respond to what we see? We must imitate the good, and carefully avoid the bad. The one who does good is "from God." The reader may note Diotrephes' bad example, but is clearly commanded to follow what is good.
v12: Demetrius was another good example. He was blameless before the saints. The truth commends such a person, for he comes to the light, proving that his deeds have been done in God (John 3.21).
v13-14: The conclusion, leaving questions in our minds; Was Diotrephes dealt with? How did the church develop? Was there a 'happy ending'? We do not know. But the command stays ringing in our ears, to walk in the truth.