The words ineffective and unfruitful (or unproductive) basically have the same meaning. Peter is thus saying that Christian believers will be useful to Christ if they make a diligent effort to grow in moral character. Without such effort they are like idle workers wasting their day in the marketplace (Matthew 20:3, Matthew 20:6).1 The result will be a life of vice (Jude 1:12; Matthew 13:22; Mark 4:19; Ephesians 5:11), the opposite of a life filled with good deeds (Titus 3:14).2 In contrast, if one grows in virtue, then the knowledge of Christ mentioned in 2 Peter 1:3—the coming to know Christ that happens in Christian conversion—will not be unproductive or unfruitful.3
8 For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ.