The Heidelberg Catechism is a Protestant confessional document taking the form of a series of questions and answers. It is commonly used in teaching Reformed Christian doctrine and is widely regarded as one of the most influential of the Reformed catechisms. The Catechism is divided into fifty-two sections, called "Lord's Days," which were designed to be taught on each of the 52 Sundays of the year. The Synod of Heidelberg approved the catechism in 1563.