Yes she was, but in her own way. She believed in a single God. Believing was something private for her. She didn't share her beliefs with anybody. She didn't go to church or did any other community praying. She made her own interpretation of the religious rules. This suited her character as Barbara didn't like to be told how she had to lead her life, not even by a vicar or priest.
For Barbara believing was seeking comfort in times when she was lonely or feeling down. I think she wrote in her diaries about her prayers, but I couldn't read the diaries, therefore I am not sure.
Only once she asked me to pray with her. That was on the day we got engaged.
(thanks to Natalie for the translation)