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The Institutes of the Christian Religion (Institutio Christianae religionis) is John Calvin's seminal work on Protestant systematic theology. Highly influential in the Western world and still widely read by theological students today, it was published in Latin in 1536 (at the same time as the English King Henry VIII's Dissolution of the Monasteries) and in his native French in 1541, with the definitive editions appearing in 1559 (Latin) and in 1560 (French).

The book was written as an introductory textbook on the Protestant faith for those with some previous knowledge of theology and covered a broad range of theological topics from the doctrines of church and sacraments to justification by faith alone and Christian liberty. It vigorously attacked the teachings of those Calvin considered unorthodox, particularly Roman Catholicism to which Calvin says he had been "strongly devoted" before his conversion to Protestantism.

The Institutes is a highly regarded secondary reference for the system of doctrine adopted by the Reformed churches, usually called Calvinism. (from wikipedia.com)