Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Christĭānismus, i, m., = Χριστιανισμός, Christianity, Tert. adv. Marc. 4, 33; Aug. Civ. Dei, 19, 23, 1; Hier. in Gal. 6, 4.