。
She was a Christian during her lifetime.
Literal
She [topic-は] alive-during Christian was.
存命中 ('during one's lifetime, while alive') is a formal/literary set phrase, often used in writing about deceased people — the implicit context is that the subject is no longer alive. キリスト教徒 ('Christian person, Christian believer') is built from キリスト ('Christ,' a katakana loanword from Portuguese 'Cristo') + 教 ('religion') + 徒 ('disciple, follower'). であった is the formal/written past copula, more elevated than だった.