Beloved, it is a faithful thing you do in all your efforts for these brothers, strangers as they are, who testified to your love before the church. You will do well to send them on their journey in a manner worthy of God. For they have gone out for the sake of the name, accepting nothing from the Gentiles. Therefore we ought to support people like these, that we may be fellow workers for the truth. – 3 John 1:5-8 ESV
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 is talking about here are likely itinerant missionaries who are traveling throughout the country spreading the gospel, similar to what Paul did. They have come to the church where Gaius served and he welcomed and supported them, 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.