Catholic to Protestant
John Knox was ordained in the Catholic church sometime between 1530 and 1540. He first publicly professed the Protestant faith in 1545. The immediate instrument of his actual conversion was probably George Wishart, who, after a period of banishment, returned to his native country in 1544, to perish, within two years, at the stake, as the last and most illustrious of the victims of Cardinal Beaton.
While residing in the castle of St.Andrews, a stronghold and place of refuge for many Protestants, in July of 1547, the castle was seized by outside forces and John Knox became a French galley-slave for nineteen months. There he experienced hardships and miseries which are said to have permanently injured his health.
Due to the troubling times in Scotland, Knox went to England in 1549 and preached the Bible until the reign of Queen Mary I (1553 to 1558, a.k.a. Bloody Mary), during which time he removed himself to Frankfort Germany. There he came under the influence of John Calvin. He eventually returned to Scotland after several years in Geneva, and began preaching against the Papal Church. Knox was arrested under Queen Mary Stuart of Scotland in 1560 and tried for treason, but was acquitted. He spent his remaining years preaching and lecturing in Edinburgh and St. Andrews.