What is the cure for the obsessive doubt (waswas) about falling into disbelief (kufr) and apostasy (riddah), and what is the ruling on one who falls into disbelief unknowingly?

The treatment for obsessive thoughts is to turn away from them, ignore them, and not pay them any attention. So, strive against yourself to abandon them. Know that you have not left Islam because of these obsessive thoughts. Do not recite the Shahada with the intention of re-entering Islam, and do not repeat any of your prayers. If you strive to learn the rulings of the Sharia and study the correct creed, you will come to know the nullifiers of Islam and avoid them without extremism or obsessive thoughts. And whoever commits one of the nullifiers of Islam out of ignorance is excused for their ignorance.