An Acceptable Time by Madeleine L’Engle

A flash of lightning, a quiver in the ground, and, instead of her grandparents' farm, Polly sees mist and jagged mountains—and coming toward her, a group of young men carrying spears.

Why has a time gate opened and dropped Polly into a world that existed three thousand years ago? Will she find her way back to the present before the time gate closes—and leaves her to face a group of people who believe in human sacrifice?