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).
Due to the popularity of the saint, the name became common in the Christian world during the Middle Ages. The Normans brought it to England, where it was commonly spelled Cecily — the Latinate form Cecilia came into use in the 18th century.
As an English name it can also derive from the month of August, which was named for the Roman emperor Augustus.