The devil works on legalities

Because I say the devil works on legalities, it doesn't mean I'm saying we are justified by the keeping of the Law of Moses. An example of trying to be justified by the Law of Moses would be, "I've kept all the commandments, so now I should be allowed into heaven".

An example of the devil working on legalities can be found in many scripture verses. Here's one:

“Be angry, and don’t sin.” Don’t let the sun go down on your wrath and don’t give place to the devil.  - Ephesians 4:26

The devil looks for footholds. If you become angry with someone and don't forgive that person, you're giving the devil a foothold in your life.