“9 For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins.” (2 Peter 1:9–ESV)
The qualities that Peter recorded in verses 5-7 are conspicuously needful. To lack them as a mode of operation, a life-shape is to forget the whole point of salvation. Salvation is the result of repentance from past shortcomings, sins, rebellion against one’s maker. When one is weighed down (conviction is the church word) by awareness of guilt to the point of confession and repentance that is to be a beginning. “Man that is bad, God please forgive me.” That is a bit of how it goes. The natural next step is a life of godliness. It is the ladder of maturity from faith to love. When these traits are not evident it is as if they have forgotten that day when they abandoned their old ways.
Peter construes this as being nearsighted and that to the point of being blindness. Seeing is for observing: danger, people, beauty, environment. To be blind is to lose that. If you do not have good vision then you will not see the pit in the road and you will fall into it or you will break your axle or throw out the alignment on your vehicle and go astray.
The bad ladder (the inverse of verses 5-7)
After a fashion one can construct a “bad ladder” by turning or inverting the things of verses 5-7. Doing that results in the following: if one is bad, stupid, indulgent, prone to quit, irreverent, inhospitable, and/or unloving then they are spiritually myopic, spiritually blind (depending upon how much you lack these qualities).