It only takes one wealthy wackjob to buy a chemical or nuclear weapon and use it to kill millions of people.
No, it also takes a seller of such weapons. And there aren't any, or we'd have been sweeping up the remains of some city, political center, or major chunk of infrastructure by now. The whole "terrorists and nuclear weapons" is a total mind job done on you and yours by your government. One thing to to keep in mind: Nukes are very difficult and expensive to manufacture, and pretty damned difficult to lose track of.
Civilization isn't likely to die due to nuclear weapons. We've set off well over a thousand of them already, and there's no particular notable effects other than the low hum of hysteria at the intersection of the set of the ill-informed and the paranoid.
Also, Chemical weapons are a lot less "mass" than nukes are, barring very sophisticated delivery systems, which again, aren't available to religious tools. Bacterial weapons are vaguely possible (although still very, very technical), but incorporate the downside of most likely eventually killing everyone everywhere instead of just the target(s), and so not even your average superstition-addled dingbat seriously considers them.
If you are a US citizen, If you want to worry about civilization, you should be worrying about the decay of our government from one authorized by the constitution into a form exclusively controlled by corporate and political groups. Because unlike the "nuclear threat", said decay is real and ongoing and has already screwed things up immensely: almost 100% loss of manufacturing capacity and so also jobs, crippling inflation, loss of citizen's rights, usurpation of article five powers by the judiciary, illegal legislation that spans almost the entire bill of rights to ex post facto laws to the complete inversion of the commerce clause, promulgation of multiple very expensive, ultimately useless wars... the problem isn't terrorists. The problem is our federal government. The whole terrorist thing is to keep the citizens looking the wrong way.