As an English name, Caleb came into use after the Protestant Reformation. It was common among the Puritans, who introduced it to America in the 17th century.
Geoffrey based parts of Merlin's character on Myrddin Wyllt, a legendary madman and prophet who lived in the Caledonian Forest. Other parts of his life were based on that of the historical 5th-century Romano-British military leader Ambrosius Aurelianus (also known as Emrys Wledig). In Geoffrey's version of the tales and later embellishments Merlin is a magician and counselor for King Arthur.
It has been used as a Christian given name in the English-speaking world since the Protestant Reformation. A famous bearer was Nathan Hale (1755-1776), an American spy executed by the British during the American Revolution.
This was a common name in classical Greece, and, due to both the saints who carried it and the favourable meaning, it came into general use in the Christian world, being especially popular among Eastern Christians. It was however rare in Britain before the 19th century. Famous bearers include three tsars of Russia (in the Russian form Fyodor) and American president Theodore Roosevelt (1858-1919).