C.S. Lewis used the name Jack for himself after childhood.
Lewis was christened Clive Staples Lewis, but he began calling himself “Jacksie” after the death of a childhood dog named Jacksie. Over time, the nickname was shortened to “Jack,” and friends and colleagues generally used that name for the rest of his life. He did not usually use “Clive” in personal conversation.
The origin of the nickname is connected to Lewis’s childhood in Belfast. His brother Warren was often called Warnie, another familiar name that remained with him into adulthood. The brothers stayed close and later collaborated on family and military history interests.
Readers sometimes assume that “C.S.” represented the name Lewis commonly used. It did not: the initials stand for Clive Staples. The author’s public byline was C.S. Lewis, while “Jack” belonged mainly to his personal identity among friends, family, and colleagues.