The OED on "-ism" has:Without meaning to get too pedantic, an a-theos ist would be one who actively does non-god, it isn't a passive state of non belief or adoxia.
-ist, which comes from the Greek suffix -istes, forms agent nouns, that is, nouns that denote someone who does something.
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Forming the name of a system of theory or practice, religious, ecclesiastical, philosophical, political, social, etc., sometimes founded on the name of its subject or object, sometimes on that of its founder. Such are Alexandrianism, Arianism, Arminianism, Brahmanism, Buddhism, Calvinism, Catholicism, Chartism, Christianism, Congregationalism, Conservatism, Epicureanism, Judaism (a1500), Latitudinarianism, Liberalism, Machiavellism, Muhammadanism, Platonism, Positivism, Presbyterianism, Protestantism, Puritanism, Puseyism, Quakerism, Quietism, Radicalism, Ritualism, Romanism, Socinianism, Taoism, Toryism, Wesleyanism, Whiggism.
"Atheism" is the practice of holding the "without god" position. That does not mean that it needs to be a positive affirmation of the absence of God any more than Protestantism is the positive affirmation of the existence of Protestants.