The Middle Ages were pretty darn sexist when it came to names! Not always, but often, they'd give a girl a male name. Then the priest, usually the only literate member of the local community, would show off his Latin skills and reason that if they wanted to name a child Philip, that would be Philippus in Latin but this child was female; so he'd enter it in the records as Philippa and use it when christening the baby, but that would be the last she'd hear of it. She'd just be known as Philip. Same with other names, obviously; Philippa works well as an example because the female form is in use nowadays.
Similarly, my guess would be that there'd be little medieval Christianas running around, never dreaming that their name wasn't Christian.
In Pilgrim's Progress, the pilgrim's name is Christian and his wife is Christiana. Heavy symbolic stuff! And Pilgrim's Progress was the Number One Sunday-school prize book in the 19th century ... which is probably why the two great-grandmothers that I know about in my family, both from the north of England, were named Christiana. Each one was known as Chrissie.