2 Peter 1:8 For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ.
This is a conditional promise: if these these things are in you, then you will not be empty and useless, ”barren and unfruitful.” Put positively, He promises that you will be full of God’s life and you will bear abundant fruit. It is the knowledge of Christ that will work to produce such a fruitful life. To ”know” Jesus is to believe Him. It is to have a living and loving relationship with Him, to be in love with a real person. This person living inside makes you live a beautiful, fruitful life.
That’s the promise, but there is a condition that causes such living: ”if these things be in you.” These things are given in verses 6-7: faith, virtue, knowledge, self-control, steadfastness, godliness, brotherly affection, and love. God is in you, and so these qualities are in you. You don’t produce them and make them happen; they are given by grace. These things are in you; therefore, you are fruitful. “Abide in the Vine, and you will bear much fruit.”
The Holy Spirit gives faith through the hearing of the gospel. Then He adds these personality characteristics to your faith so that you look like Christ. We repent and believe the gospel promises and we will be fruitful. Thank God!