What is the main conflict in Knowledge of Angels by Jill Paton Walsh?

Knowledge of Angels traces the stories of two outsiders, who change the medieval Mediterranean island of Grandinsula forever. The outsiders are discovered at about the same time. Their fates are intertwined, but they never meet. The novel is set in the time of the religious intolerance of the Spanish Inquisition, and the island is deeply affected by an Inquisitor, who comes to the island to flush out heretics.