For if anyone is a hearer of the word and not a doer, he is like a man who looks intently at his natural face in a mirror.  For he looks at himself and goes away and at once forgets what he was like. (ESV)
In our modern age people love to take selfies. Just about everyone, even children, have expensive cell phones that have high powered cameras that will show us every detail of whatever it is that we are taking a picture of. Imagine how odd it would be for a person to take a selfie, look at it, and then immediately forget what they looked like.
When James wrote this epistle there was obviously no such thing as cell phones. In fact, mirrors were not a popular item that many people owned. Poor people did not own mirrors. So, it was difficult for someone to have a true idea of what they looked like. But the wealthy did have mirrors. Dr. John MacArthur contributes the following:
“First-century mirrors were not glass but metallic, made of bronze, silver—or for the wealthy—gold. The metals were beaten flat and polished to a high gloss, and the image they reflected was adequate but not perfect.”