Australia is a founding member of the UN. Unless the UN fell (which is unlikely, being that every major world power besides Russia and China are in it), then anyone trying to fight Australia would likely have to deal with the combined might of UN's strongarm- NATO. And NATO includes America's very sizable military.

The US has long supported countries under threat from communist rule- Korea (which we halfway succeeded at) and Vietnam (though we did fail at that, more because of publicity than anything) are a couple examples. We've been in a constant Cold War up until the downfall of the USSR because they wanted to expand and spread communism, and we fought very hard to have that NOT happen.

The US supports South Korea's independence from the much less democratic North Korea, so why would we, and therefore NATO, allow a founding member of the UN come under annexation of a communist nation?

Nations don't just go peacefully under another nation's banner just to save lives. North Korea is constantly threatening their significantly smaller (militarily speaking) neighbor, South Korea, with ballistic missiles and the like. If just to save lives- why don't one of them give it up to the other? Governments don't simply bend a knee to another unless there's literally no way for them to win. Japan knew they were going to lose the Pacific War against America- we were winning territory every month, pushing them back into their home waters, even though they practically destroyed our most powerful ships at the beginning- and very thoroughly. Japan didn't surrender until we were on their doorstep, preparing a massive invasion, and AFTER we dropped experimental warheads on Hiroshima and Nagasaki. Australia is hardly like Japan, but the point still stands- great bodies of power- a government- will not bend knee to someone or something else unless there's extremely few other options.

Like I said, you do you, man. If you wanna go ahead with it, go ahead with it. I'm just saying that if you look at it logically, reasonably, there's no real way for China to conceivably want JUST Australia.

Another thing is that religious radicals are extremely looked down upon- ISIS is an example of a religious sect that's gotten way too radical. Entire countries, some of which have nothing more to do with ISIS than having the same skin color, are being practically persecuted by western nations, and in our case- cut off. If Christians were to do the same and overthrow the Chinese government- provided that the Chinese government doesn't kill them all first, if what you're saying is true (if they're fine executing them as they find them, then they won't bat an eye at killing all they can find if they start rebelling)- they'd be labelled radicals, and only China's economic importance would be enough to save its relations with other nations. More than that, China would probably be thrown into chaos with such a rebellion, because of how big it is, how many people there are, and how many of those people simply don't know what's going on or even if they care.