“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.”  
2 Peter 1:8

He told us what to expect from people who had the virtues listed in verses 5 to 7; they would be active and productive in what they know about and in the Lord Jesus. Those virtues would make them productive for life.
“But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins.”  2 Peter 1:9
This begins to make sense when you realize that there are some Christians that are almost not different from unbelievers in their manner of thinking, talking, actions and most especially what they think about God and their relationship with him, he gives us the diagnosis; he says because they lack these things- those virtues are inherent in every Christian but there must be the diligence to manifest them- they would definitely forget they were purged from their old sins.
The man is born again but never studies his bible, never uses his faith, there’s nothing pious about him, he’s erratic and sense driven, it is only a matter of time before he becomes sick and broke (because he doesn’t use his faith), immoral (because he’s not pious), arrested (because he’d fight with someone for lack of love, self control and patience), confused and either doubt his being born again or dabble into “…profane and vain babblings, and oppositions of science falsely so called:”  1 Timothy 6:20 (because he never studied his bible).
The good news is mercy triumphs over judgment, meaning, the first reason why the word of God talks about those who lack those virtues is not to judge them unto condemnation but to tell them they can change! All they have to do is to abound in faith, excellence, knowledge, temperance, patience, godliness, brotherly kindness and love and instantly that man that has been in a mess would begin to see unstoppable results! Those virtues would make him for life!