For the love of Philae - Christian Jacq

It's the 6th century A.D. and Christianity has been imposed throughout Egypt. On the holy island of Philae, off the southern coast, the last community faithful to the old ways struggles against the invader and continues in active worship in its temple.
Isis, leader of the community, is determined that their ideals will not die out. She embarks upon an unequal struggle against the all-powerful bishop Theodore to preserve the ancient mysteries. With the help of Sabni, who is bound to her by an everlasting love, Isis succeeds in restoring confidence to the followers.
However, from north to south, dangers rise up to threaten the temple's existence. For the love of Philae, the fate of a thousand-year-old faith is at stake.