Have you ever worked out and hit a plateau? Do you want to be useless or unfruitful for Christ? Of course not. 2 Pet 1 tells us to work on our character, starting with diligence, then moral excellence, knowledge, self-control, perseverance, godliness, brotherly kindness, and finally love.  Those are all great. But God follows it up with something critical, saying,

"For if these qualities are yours and are increasing, they render you neither useless nor unfruitful in the true knowledge of our Lord Jesus Christ." (v8)

We need to have these qualities, but God says that they MUST be increasing for us to be useful and fruitful. God does not provide for a state of "acceptable plateau" in spiritual growth, because He only sees two states: growing or dying. Christ is a force so powerful that we need not plateau if we don't want to. Let's encourage those around us who have plateaued to "excel still more." (1 Thess 4)