[Knillagrarp]

What I need to know is, do we have to understand everything to be true believers?

No. You do not have to understand everything to be a true believer.
You have to make your decision much before that.
Much before you understand everything - if that is possible at all.
You have to decide with partial knowledge - after all faith is about unseen.

However , deep down this free will is just an illusion and religion has nothing to do with this illusion.

This is slightly problematic. We, , have the precise expression for the corresponding ideas. This is there in the last two or three Verses of Surah Takweer. In few words our free will is thoroughly subordinate to Allah (SWT)'s Will.

Whatever we do out of our own choice is already in the Knowledge of Allah and written in The Tablet. However, His Knowledge does not compel us to perform the acts.

Wassalam,
I hope this was of assistance to you.

Nice points akhi.
My impression is that looking at the problem, over the time, from different angles should lead one to the target.

What is Calvinism?
cal·vin·ism/ˈkalvəˌnizəm/
Noun:
The Protestant theological system of John Calvin, which emphasizes the irresistibility of grace and the doctrine of predestination. Surah Takweer 81: 27 - 29
It is not except a reminder to the worlds.

For whoever wills among you to take a right course.

And you do not will except that Allah wills - Lord of the worlds. They do look the same, as brother Equalizer says.

But it covers only the second part of the assertion in last two Verses of Surah Takweer.
It leaves out the first part - that we have limited free will.

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I'll jump onto quantum mechanics later on. Lord Most Compassionate Willing.