depends what your delusions are, but believing them as truth can make them real. If you believe everyone hates you and act on that delusion. you hate them back, are mean to them, avoid them and such then your delusion might start to become true. At that point it's really a self fulling proficiency though. Another example would be if you think someone is going to fight with you about something, you go in with that attitude and are confrontational you might start a fight and be right.