John Calvin's international influence on the development of the doctrines of the Protestant Reformation began at the age of 25, when he started work on his first edition of the "Institutes of the Christian Religion" in 1534 (published 1536). Alongside the work he contributed to confessional documents for use in churches, Calvin's beliefs and practices left a direct influence on Protestantism. He is only one of many people to influence the doctrines of the Reformed churches, but he eventually became one of the most prominent theologians.