C.S. Lewis used the nickname Jack after childhood.
Lewis began using “Jack” after the death of his childhood dog, Jacksie, when Lewis was about four years old. He reportedly declared that he would henceforth be called Jacksie, a name that later shortened to Jack.
The nickname became his normal personal name. Friends, colleagues, and members of the Inklings generally called him Jack, even though his published works appeared under the initials C.S. Lewis.
“Jacksie” is sometimes given as the answer because it was the original form of the childhood name, but “Jack” is the enduring nickname associated with the adult author. “Clive” and “Staples” were parts of his given name, not the name by which he was ordinarily known.