The phrase "to leave the faith" means to stop believing in and practicing a particular religion or religious doctrine.
Sentences with «to leave the faith»
Only by leaving faith behind can one truly be free. (nakedpastor.com)
I'm currently trying to understand how I can approach careers involving the financial markets that don't force me to leave my faith at home. (alephblog.com)
All candidates should stick to politics and leave their faith out of it. (religion.blogs.cnn.com)