In 1 Peter 1:5-7, the writer has described a virtuous life. But the one who lacks these qualities has forgotten they are cleansed of sin (v.9). Two metaphors are used to describe someone that lacks the virtues listed in verses 5-7.
First, they are like people who have poor vision. They are not blind, but so nearsighted that they might as well be blind. Perhaps we might call that “legally blind” in contemporary culture. If you really cannot see well, you find yourself in difficult, embarrassing, or potentially dangerous situations. It is one thing to not be able to read a menu in a restaurant, for example, and quite another to not be able to tell one person from another. Worse yet, if you cannot see well enough to read street signs, driving a car becomes very dangerous.
In the same way, someone that is not developing godliness does not have the spiritual vision to recognize dangers around them and may find themselves not just in an embarrassing situation, but a spiritually dangerous place. A Christian who lacks self-control may say something that is harsh or judgmental when they ought to have controlled their tongue (this is something I have done many times!) They might even be too blind to know that their harsh speech is doing more harm than good.
The second metaphor is forgetfulness. A person who is not perusing godliness has simply forgotten what they are, a forgiven sinner. Most people have had a moment when they forgot something important. Usually it is a name, or something that you said you would do, etc. Like blindness, forgetfulness can lead to embarrassment (who are you again?) but also to potential danger. Think of people who have serious problems remembering who they are, such as an amnesiac or a person suffering from Alzheimer syndrome. It might be dangerous for a person to be on their own because they have forgotten critically important information that will keep them out of danger.
The person who is not pursuing godliness in Peter 1:9 has forgotten the most important thing imaginable, they have forgotten that they have already been cleansed from sin. Imagine a very dirty child who takes a bath so that they are ready for bed, and then wants to go out and play in the mud again. The believer has already been cleansed of sin, why would they “forget” and go back to their past sins?
This too might be a hint at the problem Peter needs to address in the letter. The opponents claim to have superior godly knowledge of God, and that knowledge allows them (or so they claim) to behave in any way they choose. They are “sinning that grace may abound,” to use the Pauline phrase, since later in the letter Peter will say that the opponents are twisting Paul’s letters in order to support their sin.
Once again I think Peter’s letter is applicable to the current state of the Western church. In our pursuit of some good things, we have lost sight of the most important elements of the Gospel. What is the Church blind to? What are we not seeing (or worse, closing our eyes to?) What have we forgotten in the Gospel?
Is it possible by forgetting what is really important we have become like a child who wants to go play in the mud again?