- If left to their own devices, people are naturally leaning towards a black/white dichotomy of dualism versus incompatibilism, i.e. they believe that the only possible meaning of "free will" is some kind of supernatural or libertarian free will that allows people to do whatever they want independent of their genes, upbringing, personality, current state of brain chemistry, and whatever other influence of the natural environment you may want to add. In other words, when you say "free will" everybody will assume you believe in supernatural stuff.
- If one could only convince people of the truth of determinism and incompatibilism, if they could only be convinced that there is no free will, they would become better people. For example, revenge would not make sense any more if it were understood that a criminal is the sum of their genes and experiences and thus never had any really free choice in committing their crime.
- If one could only convince people of the truth of determinism and incompatibilism, that would be a mighty blow to religion, at least partly because religious apologists use free will as a standard defense for the so-called problem of evil.
As has been pointed out by Hume in his Inquiry long, long before the much-hyped results of contemporary neuroscience were available, people are demonstrably determinists in practice. We can ask ourselves, how would people behave if they really were naturally dualists, in other words if most of us believed in "contra-causal free will" that allows others to make decisions completely independent of their intrinsic character and experiences?
Well, clearly people who really believed that would have no way of predicting how anybody else will act at any given moment. They would have to spend their entire lives curled up under a blanket at home because they would have no way of arriving at the conclusion that it is unlikely that their best friend will unprovokedly murder them. They would have no way of assessing the likelihood of a shopkeeper selling them something versus the shopkeeper punching them in the face.
Conversely, the fact that everybody who is not literally insane infers the likely behavior of others from their known character traits and from previous interactions proves conclusively that everybody who is, again, not insane really does believe that people's actions are determined by genes and environmental influences. QED.
So perhaps it is more a case of having to point out the intellectual inconsistency between somebody's everyday belief in determinism and their professed armchair-reasony belief in supernatural free will than of having to get them to believe in determinism in the first place. But the point is here, regardless of their religious or ideological attachment to dualism or supernatural free will as a disconnected intellectual exercise, through accepting in practice that people are constrained by their inherent character, childhood experiences, education, habits etc they are already predisposed to accept things like re-socialization and deterrence oriented penal laws instead of vengeance oriented ones - to the degree that lofty philosophy of mind will ever be able to overcome emotion in these matters.
For the question whether people, once they are determinists, are either compatibilists or incompatibilists, we actually have empirical data. The researchers who wrote the linked paper have carefully asked participants questions about moral responsibility under certain scenarios to figure out if they are naturally leaning towards incompatibilism or not, and it turns out that people are indeed much more instinctively compatibilist determinists than common wisdom will have it. Even when explicitly assuming that actions are predetermined by physical laws, people still predominantly see criminals as having responsibility for their actions.
What I understand least is the third assumption: That the realization of free will being an illusion would somehow be an argument against theistic religion. Yes, the free will defense against the problem of evil is a thing (not that I consider it even remotely convincing). But there are many other religious people who can happily live without a belief in free will, and if more believers become convinced that supernatural free will does not exist then their ranks will probably only swell.
I mean, that is kind of the point of religion, right? You can believe this, you can believe that, it does not have to make sense. Indeed the entire meaning of faith is to believe in something that you do not have any good reason to believe in. So if somebody wants to believe in a god, they can easily rationalize both determinism and supernatural free will to be part of that belief.
We have likely all met some religious determinists. To them, god is all-powerful and, crucially, all-knowing. And that means that they believe in determinism at least as strongly as a philosophical materialist who is convinced that natural laws and the distribution of particles 13 billion years ago already predetermined everything that would happen today, only in their case things are predetermined because god knew what was going to happen today 13 billion years ago.
Now our interpretations of these beliefs can differ. Maybe some will find solace in the thought that a powerful father-figure has worked everything out for the future because they assume that surely he will have a wise and benevolent plan in store for us. Perhaps others will find the idea of a cosmic tyrant predestining us to hell or heaven before our birth, without any chance of changing one's fate through one's free actions, plainly horrific. But here is what we cannot do: Expect religious believers like that to be impressed when we tell them that free will is an "illusion". Because their reply would be, yes, so what?