@drq every popular belief I recall commonly labeled a religion relies on explicit propagation to survive. Pessimism is so simple that it doesn't need it, it easily emerges from pretty basic ideas almost spontaneously. It does fill some very similar roles like coping with complexity of the world, but I wouldn't go as far as calling it a religion.

The best justification for pessimism I've heard in one of the Mass Effect games:
"I like to expect the worst. There's a small chance I'll be pleasantly surprised."
(Bonus points if you read this in the right character's voice in your head)

1. It explicitly paints pessimism as a personal choice
2. It provides a rationale for that choice
3. It hints at when it's effective
4. Concise af