The Kuzari: Sefer haKuzari

The Kuzari is one of the most famous works of the medieval Spanish Jewish philosopher and poet Rabbi Yehuda Halevi, completed around 1140. Its subtitle declares it to be "the book of refutation and proof on behalf of the most despised religion", which shows its purpose and context in medieval Jewish thought.
Divided into five parts, it takes the form of a dialogue between the rabbi and a pagan, in this book the pagan was the then mythologized by Jews pagan king of the Khazars who was invited to instruct him in the tenets of the Jewish religion.