This one is a little strange. You have 混む (To Be Crowded), 混ぜる (to mix), 混ざる (to get mixed), and now this: 混じる, To Get Mixed Up In. It makes sense because it has to do with mixing, but how to differentiate from the rest? Well, for this one just imagine Jesus (じ) getting mixed up in a pot of soup. He's getting mixed up in this giant pot of soup because someone thinks that if they eat him, they'll gain his powers.
The reading is the same as (almost) all the other 混 verbs. All about mamamamama!
The message from the demon is a lie, but he will mix lies with the truth to confuse us.