2 Peter 1:8

We must continue to grow in these qualities as well as possessing them; we must grow in grace (2 Peter 3:18). Failure to do so will make us "useless" (Gr. argous) in God’s hands as His tools in the world (cf. James 2:20; cf. Matthew 20:3; Matthew 20:6), and "unfruitful" (Gr. akarpous) as communicators of His life (John 15:2; John 15:4; cf. Mark 12:12-14; Mark 12:20-26). This is so even though we have received everything necessary for godly living through the knowledge of Christ (2 Peter 1:3). "Render you neither useless nor unfruitful" is a litotes, a figure of speech that affirms an idea by denying its opposite.

"’Idle’ (argous) is literally ’unworking’; it is not a picture of one unavoidably unemployed but of one who avoids labor for which he should assume responsibility." [Note: Hiebert, Second Peter . . ., p. 56. Cf. Matthew 20:3, 6.]

When we diligently add these virtues to our lives we will be both useful and fruitful, and we will evidence true knowledge (Gr. epignosis) of our Lord Jesus Christ. True knowledge of Him involves not just intellectual understanding then, but knowledge that comes through obedience. This growth should be the goal of every believer (cf. 2 Peter 3:18).

"Some of the most effective Christians I have known are people without dramatic talents and special abilities, or even exciting personalities; yet God has used them in a marvelous way. Why? Because they are becoming more and more like Jesus Christ. They have the kind of character and conduct that God can trust with blessing. They are fruitful because they are faithful; they are effective because they are growing in their Christian experience." [Note: Wiersbe, 2:439.]