Commentaries:
Do not be misled by the word established because it can suggest that they were not already established, which they were. The next verse explains what he means.
The apostle Paul is talking about "feeding the flock," preaching "the whole counsel of God" in infinite detail. The words of God—the gospel—when understood in its broadest sense, include the entirety of the Bible. It is not confined to making a witness so that people might have this held against them in a legal sense by God—as if God were saying, "I gave you My word, but you didn't respond to it."
God's primary concern is to prepare His children to share a relationship in fellowship with Him for all eternity—everybody living exactly the same way. The "gospel" requires an infinite expansion on the bare basics that bring us to conversion so we can see the application of God's way of life in every situation possible in our lives.
John W. Ritenbaugh
What Is the Work of God Now? (Part One)
Paul wanted to be with them. Personal contact would be so much more effective than a letter, than hearing things through a second or a third person. He felt that if they could just be eye to eye, face to face, see one another's expressions and be in one another's presence, it would be so much better than hearing through some other means. For us, a telephone might be fine—it is better than nothing, better than a letter—but not as good as being there in person and being able to get the effect of the other with all of one's senses.
John W. Ritenbaugh
How to Know We Love Christ