As an English Christian name, Isaac was occasionally used during the Middle Ages, though it was more common among Jews. It became more widespread after the Protestant Reformation. Famous bearers include the physicist and mathematician Isaac Newton (1643-1727) and the science-fiction writer Isaac Asimov (1920-1992).
The Normans brought the name to England in the 11th century but it quickly died out. It was reintroduced by the German House of Hanover when they inherited the British throne in the 18th century. A famous bearer was Frederick Douglass (1818-1895), an American ex-slave who became a leading advocate of abolition.
This name was borne by a number of important saints, including Cyril of Jerusalem, a 4th-century bishop and Doctor of the Church, and Cyril of Alexandria, a 5th-century theologian. Another Saint Cyril was a 9th-century Greek missionary to the Slavs, who is credited with creating the Glagolitic alphabet with his brother Methodius in order to translate the Bible into Slavic. The Cyrillic alphabet, named after him, is descended from Glagolitic.
This name has been especially well-used in Eastern Europe and other places where Orthodox Christianity is prevalent. It came into general use in England in the 19th century.