When a person who was born a Jew decides to move away from their culture due to a desire to pursue other paths in their life. The process is simply the decision to reject the label that they were born with.
David: Dude, I fucking hate being born a Jew.
Rob: Man you should just unjewify yourself.
David: Shit yo, you're right imma do that right now
Rob: Sweet, let me know when you wanna come to church with me