With Calvinism, it's "once saved always saved", so if later you disbelieve, then what?
Then you didn't really believe and weren't really saved in the first place (note, I am quoting a serious Calvinist friend of mine on this one). A person has to persevere to their death for their faith to have been legitimate.
To keep the thread on topic, I am not debating this doctrinal invention, rather trying to show that there is a similarity between the spread of Calvinism and Islam, and show that this is part of a general phenomenon with the spread of Islam and Mormonism: part of the doctrine results in intense social pressure to conform.
In Islamic Sharia law, one must have a real, sincere conversion. Then Sharia law says that if you leave, bad things happen. The community members are all supposed to reject you at best.
That's alot of pressure to stay in Islam.
Likewise, Calvinism invented the doctrine, like you said, that if you disbelieve then "you didn't really believe and weren't really saved in the first place." In practical terms, the effect is that you have young people in Calvinist families who grow up assuming (and therefore believing) that what they were taught is true, but then they get older and have all kinds of religious doubts about the Calvinist system. "Maybe all the non-born agains aren't going to hell?", "Maybe the Orthodox are right?"
But how can they express these doubts? They have spent years telling the family they believe 100%, and now they have doubts? they question? That is not the sign of the saved, but the sign of the unsaved. There is lots of pressure in the society to rigidly conform to the Calvinist system without saying significant doubts or suggest that they could be wrong. No exploration of non-Calvinist ideas, only risk of rejection if you do so.
That is how Islam spreads- you get in, the others keep you in with lots of pressure.
That is how Calvinism spreads- if you get in, it means you believe 100%, you are chosen irresistibly, so you cannot disagree with the system without showing yourself to be one of the "Totally Depraved" and risking ostracization.
In fact, in the pure Calvinist societies of Massachusetts and Switzerland, they did kill some they labeled heretics.
I understand that Muslim societies and Calvinist groups often do not work this way in reality (see for example many liberal Muslims in the former Soviet Union and common liberal attitudes in the modern-day Presbyterian Church USA). However, that is the way the school of thoughts works. If you leave, you get punished, or if you express disagreement, you get ostracized as a non-saved who deceives by claiming to be saved.
Of course, these schools of thought are problematic, which is why many movements coming from Islamoc Sharia or Calvinism, often fail to follow it exactly, if not in theory then in practice, Inshallah - God willing!Regards