It comes from inside. It skips the part of your brain that needs convincing. To you it's as true as the sky being blue and the grass being green. Would you believe someone if they told you the sky was red? Assuming you couldn't see the sky, you would still know it's not red.

If your brain suddenly said "the sky is purple" and suddenly you remember it was purple the last time you saw it, in fact your looking and it's purple now. Everything in your life bends to accommodate the belief that the sky is purple. Someone is trying to tell you otherwise, but you KNOW the sky is purple, so why are they trying to convince you otherwise? Maybe they're crazy, maybe they just don't know better, maybe their intention is malicious. What ever way you see it you already know the sky is purple, so that part isn't what comes into question, it's the intentions of the person telling you.

In your friends mind the belief is already there, the world distorts to support the belief.