Young Remeya worships the forbidden horned god. A worship made taboo half a millennium ago. Performed still in secret by a few. Quietly tolerated by the king. Epic fantasy in which old beliefs and old loyalties clash with hidden magic in the Middle Ages of J.M. Ney-Grimm's god-touched North-lands.
The Dark Is Rising by Susan Cooper;
A Door in the Wall by Marguerite De Angeli;
Charmed Life by Diana Wynne Jones