3 John 1:8

We therefore ought to welcome such, that we may be fellow-workers for the truth.

See under 3 John 1:1:7 for three reasons why missionaries such as these should be supported, the third being, "that we may be fellow-workers in the truth," that is, participants in the rewards of spreading the gospel. John has built up the case here to show how important it was for such men to be aided, thus pointing up the sinful nature of Diotrephes' actions in shutting his doors against them and blocking the efforts any one else might have been willing to make on their behalf. All of this contrasts with the beautiful and hospitable behaviour of Gaius.