Riddah is reverting to Kufr (disbelief) after accepting Islam. It may be by word, action, erroneous belief or doubt. Consequently, whoever associates other partners with Allah; denies His Lordship, His Oneness, one of His Attributes or some of His Books or Messengers; curses Allah or His Messenger; denies any of the consensually forbidden things or renders them lawful; denies one of the Five Pillars of Islam or doubts their obligation; doubts Resurrection, the truthfulness of Muhammad (peace be upon him) or any of the Prophets or prostrates to an idol, a planet or the like, has disbelieved and apostatized from Islam. Please read the chapters discussing Riddah in the books of Fiqh (Islamic jurisprudence), as scholars have elaborated on this topic. (Part No. 2; Page No. 9) Based on the above explanation, you can understand
the verbal, practical, and doctrinal forms of Riddah.