John Chrysostom

And then, after having showed forth their virtue, he exhorts them also on their behalf, saying, "Whether any inquire about Titus; he is my partner and my fellow-worker to youward." What is, "Whether about Titus?" 'If,' says he, 'it be necessary to say any thing, this I have to say,' "that he is my partner and fellow-worker to youward." For he either means this; or, 'if ye will do anything for Titus, ye will do it unto no ordinary person, for he is "my partner."' And whilst appearing to be praising him, he magnifies them, showing them …

Homily 18 on 2 Corinthians ↗

Theophylact of Ohrid

That is, if one must say something about Titus, I will say that he is "my partner" – he helps me in teaching you and bringing you benefit. Or: if you do something for Titus, you will do it not for an ordinary man, but for my partner. That is, if you wish to hear something about others, they too have great claims on your trust; for they are our brothers. That is, they were sent and chosen by the churches. What is most important of all, he placed at the end. Everything, whatever may be in them, he says, relates to Christ.

Commentary on 2 Corinthians ↗

Thomas Aquinas

Secondly, he discloses the cause of the diligence he assumed, because Apollos is confident of you from the things Titus has said about you, commending you. Hence, he says, "because of his great confidence in you. As for Titus, he is my partner," and because he gladly went in fellowship with Titus and Luke, or Barnabas, and because he was induced to do this by the apostles of the Church, who are in Judea. …

Commentary on 2 Corinthians ↗