An Acceptable Time by Madeleine L’Engle

A flash of lightning, quivering 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 3,000 years ago? Will she be able to get back to the present before the time gate closes — and leaves her to face a group of people who believe in human sacrifice?