"Dear friend, do not imitate what is evil but what is good. Anyone who does what is good is from God. Anyone who does what is evil has not seen God." NIV translation
The key to understanding this verse is to understand what is meant by "imitate". The word means to follow which is the same thing as the meaning of disciple. It speaks of a pattern in your life and if the pattern of your life is to do evil things, then, you have not seen God. If you stumble every now and then but get back up and the pattern of your life is to follow after Christ, then, you are from God.
Now, we are introduced to the third man in the drama that was happening in this local group of believers. Evidently, Demetrius was a brother in Christ who had been sharing the gospel but was thrown out of the fellowship by Diotrephes. John tells his good friend Gaius that Demetrius was a wonderful man of God and that it was right to stand with him.
Even today, there are "church leaders" who act the same as Diotrephes and we are called to stand with those that they persecute just as Gaius did for Demetrius.
"I have much to write to you, but I do not want to do so with pen and ink. I hope to see you soon, and we will talk face to face. Peace to you. The friends here send their greetings. Greet the friends there by name." NIV translation
Although it may be hard for us to understand, John preferred to talk to people face to face in the same manner that Jesus did. In our world today, everyone wants to send a text message or anything else to avoid direct contact but you have a much more honest and meaningful conversation when it is face to face.