C.S. Lewis’s first name was Clive. His full birth name was Clive Staples Lewis, although friends and family generally called him Jack rather than Clive.
The nickname began in childhood after Lewis’s dog, Jacksie, was killed by a car. Lewis reportedly insisted that he would henceforth be called Jacksie, which gradually became Jack. The name remained with him throughout his adult life, and many biographies refer to him simply as Jack Lewis.
The initials C.S. stand for Clive Staples, not Charles Staples or Christopher Staples. Lewis used his initials professionally, especially on the title pages of his books. His middle name, Staples, came from his mother’s family and was also the surname of his maternal grandfather, Thomas Hamilton.