Because Elijah was a popular figure in medieval tales, and because his name was borne by a few early saints (who are usually known by the Latin form Elias), the name came into general use during the Middle Ages. In medieval England it was usually spelled Elis. It died out there by the 16th century, but it was revived by the Puritans in the form Elijah after the Protestant Reformation. The name became popular during the 1990s and 2000s, especially in America where it broke into the top ten in 2016.
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).
In the medieval legend of Tristan and Iseult this was the name of a king of Cornwall. It was also borne by the American author Mark Twain (1835-1910), real name Samuel Clemens, the author of The Adventures of Tom Sawyer and Adventures of Huckleberry Finn. He took his pen name from a call used by riverboat workers on the Mississippi River to indicate a depth of two fathoms. This is also the usual English spelling of the name of the 1st-century BC Roman triumvir Marcus Antonius (Mark Antony).