First thing this morning after a shower I grabbed three or four vitamins and chugged 'em down with some water. It's really something you and I do without thinking. We naturally want to take care of our bodies and do what's best for them--which includes supplementing them with vital nutrients they don't naturally create. This prevents sickness and even premature aging and death.
Peter tells us we also need to "supplement our faith". Now that's an interesting truth! Our faith, like our bodies, needs attention. It needs spiritual supplements to prevent it from withering--nutrients like Vitamin V (Virtue) and Vitamin BA (Brotherly affection). And what does Peter say these supplements will prevent? Things like "unfruitfullness", "ineffectiveness", and "spiritual blindness".
In fact those who don't practice and add these spiritual vitamins to their faith Peter says:
...is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins. Therefore, brothers, be all the more diligent to make your calling and election sure, for if you practice these qualities you will never fall. For in this way there will be richly provided for you an entrance into the eternal kingdom of our Lord and Savior Jesus Christ.
-John