Atheism is no more of an ism in the ideological sense than metabolism, bruxism, botulism, alcoholism, or euphemism are. That's the root fallacy again, a type of semantic fallacy.
Would you have like Christism better?Ya, and it's just a technicality that Christianity ends in -ity...
In Spanish, the suffix "-dad" plays the same role as "-ity" does in English. Security/safety is seguridad, trinity is trinidad, charity is caridad, loyalty is lealtad, chastity is castidad, spirituality is espiritualidad, curiosity is curiosidad, etc.