and, after looking at himself, goes away and immediately forgets what he looks like.
This is the path to self deception. Look into God's Word. It reveals who you are. You have the opportunity to change, but you don't. You will (rest assured) deceive yourself.
This is a warning. It is a promise. If you do not change, as a result of looking into the Word, you will deceive yourself.
What is that deception?
You think that your stance with God is acceptable. You are OK. You walk away thinking that you are pretty good.
If a young teenage boy goes to the mirror in the morning, realizes that he has bed-head or maybe he sees a huge blemish on the end of his nose, but walks away without doing anything about his hair or popping the zit... He may go through the day, thinking he is OK, but everyone around him can see the problems.
Something needs to be done about the problems.
Don't go to God's word without changing as a result. Maybe even when you are reading this post (not because of my words, but because of the quote from the Bible at the top) you might realize some blemishes in your own life. Be a doer and a hearer. Don't deceive yourself. If you walk away unchanged then you are self-deceived.
In Matthew 7 Jesus tells us of a large number of people who will stand before God, ready to enter in, but Christ tells them "...depart from me, I never knew you." These people respond in a state of alarm. They are surprised by this. How did they enter into this state of self-deception? Maybe it was by hearing the Word, but not being a doer.