C.S. Lewis’s first name according to his birth name was Clive.
He was born Clive Staples Lewis, and his initials became the name by which he was publicly known. Family and friends later generally called him “Jack,” a nickname that originated after a childhood dog named Jacksie was killed. Lewis adopted the nickname and used it throughout much of his life.
The initials “C.S.” can make readers assume that his first name was Charles or Christopher. Neither is correct: “Clive” was his given name, while “Staples” was his middle name. The initials also became closely linked with his fiction, apologetics, literary criticism, and Christian writing.
Lewis’s published identity was not limited to one genre. He wrote the Narnia novels, the science-fiction trilogy sometimes called the Ransom Trilogy, theological works such as Mere Christianity, and studies of medieval and Renaissance literature. His nickname “Jack” is therefore more common in biographical accounts than his birth name.