2 Peter 1:8

If there is less faithfulness, less care and watchfulness, since we were pardoned, than there was before, and less diligence, less outward obedience, than when we were seeking remission of sin, we are both slothful and unfruitful in the knowledge of Christ, that is, in the faith, which then cannot work by love.

For if these things be in you,.... Are wrought in you by the Spirit of God, and exercised and performed by his assistance, who works in his people both to will and do: and abound; increase in their acts and exercises by the frequent performance of them: they make you; both by way of influence and evidence, that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ.

For if these things be in you and abound - If ye possess all there graces, and they increase and abound in your souls, they will make - show, you to be neither αργους, idle, nor ακαρπους, unfruitful, in the acknowledgment of our Lord Jesus Christ. The common translation is here very unhappy: barren and unfruitful certainly convey the same ideas; but idle or inactive, which is the proper sense of αργους, takes away this tautology, and restores the sense.

As their subsisting in us constitutes us not barren or idle, so their abounding in us constitutes us not unfruitful in respect to it. It is through doing His will, and so becoming like Him, that we grow in knowing Him (Joh 7:17).

For if these things be in you, and abound - If they are in you in rich abundance; if you are eminent for these things. They make you that ye shall neither be barren nor unfruitful - They will show that you are not barren or unfruitful. The word rendered “barren,” is, in the margin, “idle.” The word “idle” more accurately expresses the sense of the original. The meaning is, that if they evinced these things, it would show. (1) That they were diligent in cultivating the Christian graces, and, (2) That it was not a vain thing to attempt to grow in knowledge and virtue. Their efforts would be followed by such happy results as to be an encouragement to exertion. In nothing is there, in fact, more encouragement than in the attempt to become eminent in piety. On no other efforts does God smile more propitiously than on the attempt to secure the salvation of the soul and to do good. A small part of the exertions which men put forth to become rich, or learned, or celebrated for oratory or heroism, would secure the salvation of the soul. In the former, also, men often fail; in the latter, never.