Another way to put this, is the belief that physical matter, form, body, world, is capable of CAUSING something to happen.
Elsewhere I recall, I think in the early urtext, Jesus telling Helen that anxiety must arise when you believe that illusions have the power to do something to you. Thus the belief in magic is the belief that stuff in the world is not only causing bad things to happen to you, but that you can use the same magic to undo that magic.
It should be obvious how self-contradictory that is. Trying to use a magical power outside of you in order to counteract the effects of a magical power outside of you, means that you will just continue to reinforce the belief in magic by trying to use it against itself.
Believing that the body or the world has a mind of its own is a belief in magic. It's believing that somehow the output or effects of the system have gained some kind of magical will of their own, that they've taken over, they've gone rogue and are now running the show. As though images coming out of a projector come alive on the screen and start directing their own movie.
Jesus is suggesting that believing in magic will 'cause sickness'. Ie, if you believe the world or body can affect you, that GIVES power to it and manipulates it as though it magically is doing something to you on its own. That is self attack, and it will produce symptoms of sickness.
If a person has even a tiny amount of unawareness, they are in a state of believing in magic.
Perceiving that you did not choose what has happened, and that therefore there is some other 'will 'or 'cause' separate from you, is a belief in magical causes. Perceiving that other people do things to you against your will is a belief in magic.
Magic essentially is victim consciousness.