Test. I believe my beliefs are correct because if I didn't believe my beliefs are correct then I would change my beliefs. None of us have privileged access to the truth. I cannot tell whether your beliefs are right or wrong. All I can tell is whether your beliefs are the same as my beliefs. The same applies to society and every subculture.
Suppose a group is 80% right and 20% wrong. If you copy the group's beliefs exactly then the group will believe you are 100% right even though you are 20% wrong. If you were actually 100% right then the group would believe you are 20% wrong.
It is hard to be 100% correct. Suppose you are exactly as right as the group except your beliefs were orthogonal to the group's. Then then the group would believe you to be 36% wrong (64% right) compared to the group which believes itself to be 100% right.
Being correct and appearing correct are contradictory values. You cannot satisfy both at the same time. That's why [I like Peter Thiel's question](https://www.lsusr.com/blog/list-of-unusual-beliefs.html).
Tell me something that’s true that almost nobody agrees with you on.