It’s my experience that when people believe in something, it’s more often a “heart thing” than a “mind thing.” That’s why the phrase “make up your mind” is actually incorrect. Usually someone would “make up your heart” by perceiving something to be one way or another, and then create rational reasons to support what’s in your heart.
For example, let’s say that my house is super messy and I’m trying to decide whether and how to clean it up. But of course there’s laziness preventing me from doing the cleanup. And perhaps lately I have so much stuff to do that I don’t have the time or physical energy to do it. So in my heart I just don’t feel the urge to clean my messy house. So my mind kicks in to give me reasons to justify me not cleaning things up: “No one’s visiting my house in the next few weeks anyway”… “It’s so messy that the cleanup is never going to get done, so it’s not worth doing it”… etc.
Sometimes this falls into the realm of self-fulfilling prophecy, especially when it comes to relationships. For example, if X feels that Y doesn’t love X, X can try to find reasons from Y to justify what X believes. For example, one day Y forgets to call X for a whole day. X would reason that to mean that Y doesn’t care about X and therefore Y doesn’t love X. By doing so it justifies what X believes.
Weird cults take advantage of this. You get in there and they’re so nice to you that you feel like you’re part of a bigger and better family. They paty attention to you and make you feel like a loved person. Later on, your mind reasons that what you’re doing is worthwhile as a part of that family, even suicidal acts.
I think even more tragic could be something like what happened in Nebraska, where a 19-year-old believes he’s worthless and so he goes to the mall and shot people and then killed himself, thinking that he’d now become famous. It’s not rational to us, but it really started by a heart thing and then rationalized and justified in his mind afterwards to a point where it’s so true to him.
We should seek God to guard our hearts and our minds…
“…the peace of God, which surpasses all comprehension, will guard your hearts and your minds in Christ Jesus. Finally, brethren, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is of good repute, if there is any excellence and if anything worthy of praise, dwell on these things.” (Philippians 4:7-8)