The Christ Myth Theory is an idea that Jesus Christ never existed, that all the stories and ideas about him come from other stories of gods and heroes from other cultures. This means that Jesus wasn't an actual historical figure, but rather a story inspired by religions from long ago. The idea of a "dying and rising god" is thousands of years old, from stories like Dionysus and Persephone in Greek mythology to stories from Egypt and other religions.
So, according to the Christ Myth Theory, Jesus never actually existed, but the stories and ideas about him have been around for a long time in different cultures.