Mark doesn't want to be a "cranky missionary" shaking his finger at bad American Christians but he wants to point out some areas of his concern.

v1-5
John is happy that Gaius is growing in the Lord

v6-7
John is noting that some missionaries had been sent out

v9
Diotrephes hindered apostolic authority by putting himself in a position of pre-eminence. This is the area with which Mark is concerned -- incorrect position of authority. His concern is that "missions agencies" have supplanted churches in the chain of command. "partnering"

Acts 13:2
Tells us how missionaries had been sent forth, out of/from the church.

Titus 2:11,12
the key to this passage is "the grace of God"