2 Corinthians 8:16-9:5

See, in the time when Paul was writing this, there was no "Bible Colleges" or really any way to truly know whether someone was teaching the things of Jesus. Sure they could talk with the apostles, but there was no Bible to test peoples words or no internet to look up the truth (since everything on the internet is true!). Paul is about to send Titus to the church to help spread the word and build the church. Paul is assuring the church that Titus has the credentials and he really is teaching the word of God.