The Heidelberg Catechism is a Protestant confessional document taking the form of a series of questions and answers, for use in teaching Reformed Christian doctrine. It has been translated into many languages and is regarded as one of the most influential of the Reformed catechisms.

The Heidelberg Catechism is one of the three Reformed confessions that form the doctrinal basis of the original Reformed church in The Netherlands, and is recognized as such also by the Dutch Reformed churches that originated from that church during and since the 19th century.

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. In its current form, the Heidelberg Catechism consists of 129 questions and answers.
(Summary from Wikipedia)