2 Peter 1:8

For if. Peter is not satisfied with the presence of the preceding virtues in the Christian’s life; he intends that they should flourish and increase in potency and possibly in numbers (see on ch. 3:18).

They make you ... be. Or, “they keep you from being.”

Barren. Gr. argoi, “lazy,” or “idle.” It is impossible for one who is endued with the virtues detailed in vs. 5-7 to be an idle member of the church. His faith, brotherly kindness, and love, to mention only three of the eight qualities listed, will impel him to work for others and for his Lord’s kingdom.

Unfruitful. Here the apostle includes a promise that Christian service, rendered through the operation of the basic qualities that have just been examined, will be productive. As money well invested is expected to yield dividends, as a field cultivated is expected to produce good crops, so the Christian life, well furnished with every needed virtue, is certain to produce results (cf. Phil. 1:11; James 3:17).

Knowledge. Gr. epignosis, as in v. 2 (see comment there), not gnosis, as in v. 5. The preceding virtues can be developed, applied, and find their true end only in connection with a full knowledge of Jesus Christ. If they are not fulfilled in connection with Jesus, they will fail to produce their true fruitage, but if they function in continual connection with the Saviour, their fruitfulness need know no limit.