Knowledge of Angels - Jill Paton Walsh

It is, perhaps, the fifteenth century and the ordered tranquillity of a Mediterranean island is about to be shattered by the appearance of two outsiders: one, a castaway, plucked from the sea by fishermen, whose beliefs represent a challenge to the established order; the other, a child abandoned by her mother and suckled by wolves, who knows nothing of the precarious relationship between Church and State but whose innocence will become the subject of a dangerous experiment.

But the arrival of the Inquisition on the island creates a darker, more threatening force which will transform what has been a philosophical game of chess into a matter of life and death...