913: the Umayyad rebel `Umar ibn Hafs ibn Ya`fa-r converts to Christianity from his stronghold in the Granada region. He allies with the Kingdom of Asturias and proposes to divide Al-Andalus between the two. The war goes well, and they gain territory. When the Fatimids show up, they provide aid to the Christians to undermine Umayyad / Sunni power there. By 1000, there is a much larger Asturias kingdom in the north, but also an Arabic Christian Kingdom of Granada in the south.
Note that most of the population of Al-Andalus in the early 10th century was Christian (Mozarabic), the mass conversions came after the mid-900s.