Dear friend, you are faithful in what you are doing for the brothers and sisters, even though they are strangers to you. They have told the church about your love. Please send them on their way in a manner that honors God. It was for the sake of the Name that they went out, receiving no help from the pagans. We ought therefore to show hospitality to such people so that we may work together for the truth.
In John’s previous letter, he warned the church against any kind of support or encouragement for those he called antichrists. In this passage, we see him addressing the flip side of the coin. The brothers he was talking about here were likely itinerant missionaries who were traveling throughout the country spreading the gospel, similar to what Paul did. They had come to the church where Gaius served, and he welcomed and supported these missionaries, unlike Diotrephes, who wanted nothing to do with them.
While we should not encourage antichrists, and so participate in what they are doing, we should support those who are proclaiming the true message of Christ. In supporting them, we become participants in what they are doing. You may not be called to the mission field yourself, but you can support a missionary; pray for them, write to them, offer financial support, welcome them when they visit; be a fellow worker with them for the truth.