Final answer:
Jean-Paul Sartre believes that humans determine their own essence, not God.
Step-by-step explanation:
According to Jean-Paul Sartre, a core belief of existentialism is that existence precedes essence. This means that humans first come into existence and then determine their own essence. Sartre argues that there is no pre-existent essence that makes a human what they are, and there is no God creating their essence. Instead, humans are free to choose and create their own nature and identity.