I believe that first, God's will is The Truth (everything that happens and that is, is according to the will of God).  Understanding this, I believe thus "The spirit of his mouth" to be The Truth. So the truth will be spoken and known. Devils are backwards (couldn't be further from the truth), and thus are averted by (relevant) truth. It makes them uncomfortable. These devils spend so much time speaking falsely that they believe their own lies, which translates to believing what they want to believe (their own will) rather than the Truth (putting their own will above that of God). I have a "mental disorder" in which I hear voices.  I believe these voices to be devils, and have made some observations about them and what/how they say and speak.  This is what led me to read the bible and come to the conclusions that I have concerning devils.