Why did my mother believe in God?

Kami no Ko wa Tsubuyaku

One family changes as they join a religious group. Kinoshita Haruka is forced to engage in her religious activities by her mother, Aiko, who is an avid believer and forbids her from making friends or participating in events at her school. Her father Shinji distances himself from religion and tells his daughters, Haruka and her younger sister, Inori, the importance of "believing in yourself." When Shinji suddenly falls ill, their "belief" begins to waver.