。
Mother is a Christian.
Literal
Mother [topic-は] Christianity [obj-を] faith-is-holding.
信仰する is a noun + する compound meaning 'to have faith in'; the ~ている form here marks the resulting state — one doesn't become a Christian at each moment, one is in a state of believing. English 'is a Christian' captures the stative reading better than a progressive 'is believing.' キリスト教 (Christianity) uses the suffix 教 (-kyō) that forms religion names: 仏教 (Buddhism), 神道 (Shinto doesn't use 教, interestingly), イスラム教 (Islam).