C.S. Lewis used the first name Jack throughout his adult life.
Lewis was born Clive Staples Lewis, but he adopted “Jack” as a personal name during childhood. The name reportedly followed an incident involving a childhood dog called Jacksie. After the dog died, Lewis said he would be called Jacksie, which later became Jack.
Family members and close friends used Jack rather than Clive. His brother Warren was especially important in preserving the name in private correspondence and memoirs. The nickname is also why biographies and scholarly works often refer to him as Jack Lewis.
The initials C.S. became the public form of his name as an author. “Clive Staples Lewis” remains the formal name attached to his books, but “Jack” is the familiar name associated with his personal life and friendships.