Calvinism started with what sounds like reasonable religious philosophy. If God is omniscient, omnipotent, and omnipresent, then God surely knows the end of history and whose souls will be saved and whose won’t be saved.
The original notion was closer to a piece of philosophy produced by preists who studied the Bible extensively, who likely assumed that as mere humans, they could not know the mind of God.
When this philosophy was passed to the commoner, however, the commoners decided it meant they could tell who was going to hell and who wasn’t, since it was already pre-determined, right?
At no point did anyone stop and yell “Blaspheme! It is blaspheme to claim to know the mind of God, a thing that is unknowable!” No, people just decided they could figure out who was going to hell and fuck what God thinks anyway.
It’s the same thing, it really comes down to something as simple as tribalism. They want a protected in-group but they also need an out-group to attack. It’s helpful to have a vague and nebulous enemy so any time someone wrongs you, you can instantly accuse them of being part of the shadow groups out to get you. It really prevents you from ever having to think about your own behavior much or at all, and focuses everything on a shadowy group or figure (Satan!) that they cannot even prove is there.
Is it any wonder most of these people are religious, specifically Christian, already? It’s like this kind of shit is baked into the religion.