In Revelations, God also mentions the church in Ephesus having only one bad characteristic. They have many good things going on, and they are all recognized and praised: "your labor, your patience, and that you cannot bear those who are evil" (Revelations 2:2, NKJV) , among other things. "Nevertheless I have this against you" (v4). Yes, only one thing! Why do we need to concentrate on one thing that needs to be fixed when we have so many good things going on?
Remember the story of the young rich ruler?: "You still lack one thing" (Luke 18:22), and was that one thing that kept him from following Jesus.
The one thing that is putting the well-being of the church in Ephesus at risk is that they have lost their "first love" (Revelations 2:4). Though it can be interpreted in many ways, it is obvious that the good things going on in their lives and church do not com from the same source as before. The first love could be understood as the interest in someone or something in which your whole being is genuinely and honestly involved. But the motives, obviously, can change, and that, it seems, is the problem here.
Apparently it is not enough to do good, but also why we do good. Had these people lost their identity? Apparently so, because the counsel is: "Remember therefore from where you have fallen; repent and do the first works" (v5). The solution God gives is to remember who we are, to retake our identity. Then, we can do good with the right intentions.