C.S. Lewis’s first name was Clive. His full birth name was Clive Staples Lewis, and “C.S. Lewis” was the author name by which he became internationally known.
Lewis was called “Jack” by family and friends rather than Clive. The nickname began after a childhood dog named Jacksie was killed; Lewis then adopted “Jacksie,” later shortened to “Jack.” This explains why biographies often refer to him as Jack Lewis even though his legal first name was Clive.
The initials in his public name stand for Clive Staples, not Charles Staples or Christian Staples. Lewis used the shorter form on his fiction, literary criticism, Christian apologetics, and academic work. His brother Warren was also known by the nickname “Warnie,” making informal names a recurring feature of the Lewis family story.