“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

A man who has forgotten that he was purged from his old sins, would go ahead and walk in them and the word is showing this would be the reality of any Christian that lacks the things he had mentioned in previous verses: virtue, knowledge, temperance, patience, godliness, brotherly kindness and love.
It takes faith to be born again and that faith was imparted to your spirit when you heard the gospel, now that you are born again, he expects you to add to your faith virtue, in other words seek to express the virtues; moral excellence of God that you’ve been called forth to express:
“But you are a chosen race, a royal priesthood, a holy nation, a people of his own, so that you may proclaim the virtues of the one who called you out of darkness into his marvelous light.” 1 Peter 2:9, NET
You’ve been called to show forth virtues; moral goodness, moral excellence, purity, and you can because you’ve received- and ought to walk in- the divine nature.