Learn English – the history and meaning of the suffix “-ism”

The OED is one of the most authoritative sources on this kind of thing:

Etymology: Repr. French -isme, Latin -ismus, < Greek -ισμός, forming nouns of action from verbs in -ίζειν, e.g. βαπτίζειν to dip, baptize, βαπτισμός the action of dipping, baptism. An allied suffix was -ισμα(τ-), which more strictly expressed the finished act or thing done, and which in some cases is the source of modern -ism. Besides its free use as a suffix forming verbs on ordinary nouns and adjectives, -ίζειν was (as mentioned under -ize suffix) affixed to national names, with the sense to act or ‘play’ the people in question, and hence to act like, do after the manner of, practise the habits, customs, or language of, side with or adhere to the party of, those people. Hence the noun in -ισμός had the sense of acting or doing like, siding with, adhesion to, or speaking like the people in question; e.g. Ἀττικίζειν to Atticize, to side with the Athenians, to use the Attic dialect; hence Ἀττικισμός, Atticism, a siding with Athens, Attic style of language, etc. The LXX (Esther viii. 17) and N.T. have Ἰουδαίζειν to Judaize, to live like the Jews. The derivative Ἰουδαισμός Judaism, the manner of the Jews, occurs in the LXX (2 Macc. ii. 21). The Latin Jūdaismus occurs in Tertullian (c200); Jūdaizāre in the Vulgate. Origen (a250) has Χριστιανίζειν to play the Christian, act the part of a Christian, practise Christian principles, and Justin Martyr (a150) has Χριστιανισμός the practice of Christians, Christianity. Hence late Latin chrīstiānizāre in Tertullian, chrīstiānismus in Tertullian, Augustine and Jerome. On the type of these, -ισμός, -ismus, became the ordinary ending to form names of religious, ecclesiastical, or philosophical systems; thus pāgānismus is cited by Du Cange from a council of 744. The Old French repr. of this, paienisme, paienime, painime (12th cent.) is prob. the earliest French example, and appears in English as painime, painim in the 13th cent. But, in the modern form and sense, Judaisme is found a1500, and christianisme (a1500 in French) c1525 in English. From the 16th cent. such formations are numerous.

The following are the chief uses of the suffix:

1.

a. Forming a simple noun of action (usually accompanying a vb. in -ize suffix), naming the process, or the completed action, or its result (rarely concrete); as in agonism, aphorism, baptism, criticism, embolism, exorcism, magnetism, mechanism, nepotism, organism, plagiarism, ostracism, syllogism, synchronism, volcanism. To this group in Gr. belonged asterism.

b. Applied to these, though with affinities to 2, are words in which -ism expresses the action or conduct of a class of persons, as heroism, patriotism, despotism, and the more colloquial blackguardism, busybodyism, desperadoism, priggism, scoundrelism; also the condition of a person or thing, as barbarism, deaf-mutism, orphanism, anomalism, mediævalism, parallelism; also Daltonism; with such nonce-words as bar-maidism, old maidism; all-roundism, cleverism, devil-may-care-ism, well-to-do-ism.

2.

a. 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.

These pass into terms of more or less temporary currency, as Berkeleyism, Fourierism, Jeremy Benthamism, Layardism, Owenism, St. Simonism; with nonce-words formed ad libitum, as John Bullism, Robert Elsmerism, Mahdiism; and others designating the cult of a person or family, as Bonapartism, Boulangism, Bronteism, Gladstonism, -onianism, Salisburyism, Stuartism, etc.

b. More of the nature of class-names or descriptive terms, for doctrines or principles, are agnosticism, altruism, animism, atheism, bimetallism, deism, egoism, egotism, empiricism, evangelism, fanaticism, feminism, heathenism, hedonism, idealism, imperialism, jingoism, libertinism, monachism, naturalism, opportunism, pædobaptism, paganism, polytheism, realism, romanticism, sansculottism, scepticism, stoicism, theism, universalism. These lead the way to nonce-formations of many kinds, often humorous, of which the following are specimens, chiefly from newspapers: anti-slaveryism, anti-state-churchism, anti-whole-hogism, can't-help-myself-ism, know-nothingism, Little-Peddlingtonism, L.S. Deism (after deism), nothing-arianism, 19th-century-ism, other-ism, P.R. B-ism, Primrose-leaguism, red-tapeism, Rule-Britanniaism, self-ism.

3.

Forming a term denoting a peculiarity or characteristic, esp. of language, e.g. Æolism, Americanism, Anglicism, Atticism, Devonshirism, Gallicism, Græcism, Hebraism, Hellenism, Latinism, Orientalism, Scotticism, Southernism, Westernism, etc. To these add such as archaism, classicism, colloquialism, modernism, newspaperism, solecism, sophism, witticism.

Also denoting a peculiarity or characteristic of the language, style, or phraseology of a writer, speaker, character in fiction, etc., as Browningism, Carlylism, De Quinceyism, Gibbonism, Montesquieuism, Micawberism, and similar nonce-words without number.

Adjectives pertaining in sense to ns. in -ism are formed in -istic suffix; e.g. atheism, atheistic; naturalism, naturalistic.

Draft additions June 2004

a. Forming nouns with the sense ‘belief in the superiority of one —— over another’; as racism, sexism, speciesism, etc.

b. Forming nouns with the sense ‘discrimination or prejudice against on the basis of ——’; as ageism, bodyism, heightism, faceism, lookism, sizeism, weightism, etc.

"-ism, suffix." OED Online. Oxford University Press, September 2014. Web. 20 November 2014.

so based on that entry I would say that 'magnetism' is probably from the Greek '-ισμός, forming nouns of action', possibly via Latin - let's take a look:

Etymology: < magnet n. + -ism suffix, perhaps influenced by post-classical Latin magnetismus, although evidence for this is lacking before the later 17th cent.; compare French magnétisme (1666 in sense ‘supposed occult influence’, 1720 in sense 1a), and Italian magnetismo (1684 in sense ‘mysterious fluid held to emanate from certain people’, 1698 in sense 1a).

"magnetism, n." OED Online. Oxford University Press, September 2014. Web. 20 November 2014.

tl;dr: -isms are not necessarily beliefs or worldviews. In so far as the blog implies that -ism can only indicate "a specific practice, system, or philosophy", the blog is wrong. The suffix can also be used for (among other things) 'nouns of action' e.g. mechanism, nepotism, baptism, plagiarism, etc - and this is how 'magnetism' fits in.