originally, opposition to the disestablishment of the Church of England, now opposition to the belief that there should no longer be an official church in a country
When people are asked for the longest word they know, they often say antidisestablishmentarianism.
"opposition to disestablishment of the Church of England," 1838, said by Weekley to be first recorded in Gladstone's "Church and State," from dis- + establishment in the sense of "the ecclesiastical system established by law; the Church of England" (1731). Hence, establishmentarianism "the principle of a state church," and disestablish (1590s) "to deprive (a church) of especial state patronage and support" (first used specifically of Christian churches in 1806), which are married in this word. Rarely used at all now except in examples of the longest words, amongst which it has been counted at least since 1901.