“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.”  
2Peter 1:8

In the preceding verses he talks about what things he expects to be in us to ensure our constant productivity in life:
“And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge; And to knowledge temperance; and to temperance patience; and to patience godliness; And to godliness brotherly kindness; and to brotherly kindness charity.” 2 Peter 1:5-7
We got born again when we heard the gospel, faith was imparted to our spirits and we confessed Jesus as the Lord of our lives. This means every Christians has faith:
“… according as God hath dealt to every man the measure of faith.” Romans 12:3
The word of God tells us to with all diligence, add to that faith, virtue- moral excellence. So he expects you to live a life of righteousness; to express excellence in your life once you got born again; you can’t think, talk or live anyhow.
In ensuring we walk in virtue, he tells us to sieve the thoughts we receive:
“Finally, brethren, whatsoever things are true… if there be any virtue… think on these things.” Philippians 4:8
The word says when you think right, you’d live right, and in living right, your knowledge of the Lord would produce results in your life.