2 Peter 1:8-9

For if you possess these qualities in increasing measure, they will keep you from being ineffective and unproductive in your knowledge of our Lord Jesus Christ. But if anyone does not have them, he is nearsighted and blind, and has forgotten that he has been cleansed from his past sins.

they will keep you from being ineffective and unproductive
John 15:2 "He cuts off every branch in me that bears no fruit, while every branch that does bear fruit he prunes so that it will be even more fruitful."
Colossians 1:9-14 For this reason, since the day we heard about you, we have not stopped praying for you and asking God to fill you with the knowledge of his will through all spiritual wisdom and understanding. And we pray this in order that you may live a life worthy of the Lord and may please him in every way: bearing fruit in every good work, growing in the knowledge of God, being strengthened with all power according to his glorious might so that you may have great endurance and patience, and joyfully giving thanks to the Father, who has qualified you to share in the inheritance of the saints in the kingdom of light. For he has rescued us from the dominion of darkness and brought us into the kingdom of the Son he loves, in whom we have redemption, the forgiveness of sins.
Titus 3:14 Our people must learn to devote themselves to doing what is good, in order that they may provide for daily necessities and not live unproductive lives.

in your knowledge of our Lord Jesus Christ
2 Peter 1:2 Grace and peace be yours in abundance through the knowledge of God and of Jesus our Lord.

But if anyone does not have them, he is nearsighted and blind
1 John 2:11 But whoever hates his brother is in the darkness and walks around in the darkness; he does not know where he is going, because the darkness has blinded him.

and has forgotten that he has been cleansed from his past sins
Matthew 1:21 "She will give birth to a son, and you are to give him the name Jesus, because he will save his people from their sins."
Ephesians 5:26 ... to make her holy, cleansing her by the washing with water through the word, ...

Scriptures taken from the Holy Bible, New International Version® NIV®, Copyright© 1973, 1978, 1984 by International Bible Society. Used by permission. All rights reserved worldwide.