What makes something a blameworthy religious innovation (bid'ah)

Answer

There were two general approaches among the earlier scholars to understanding bid'ah. (1) A newly invented matter may be good (hasanah/mustahsanah) or bad (sayyi'ah/mustaqbahah): the former aligns with the teachings of the religion established in the Qur'an and Sunnah, the latter contradicts them -- the approach of Imam al-Shafi'i, al-Khattabi, and many others. (2) Newly invented matters fall under the general rulings of the Shari'ah -- obligatory, encouraged, discouraged, prohibited, or generally permitted -- the approach of al-Qadi 'Iyad, al-Nawawi, 'Izz ibn 'Abd al-Salam, and many others. Additionally, something may be prescribed for itself (like the miswak's specific plant) or for the meaning it encapsulates (cleaning the mouth). If prescribed for itself, replacing it can be a blameworthy innovation *if* the justification contradicts the religion in substance or principle -- but if simply because one lacks access, cost is prohibitive, or one is accustomed to a toothbrush, it is not. If prescribed for the meaning behind it, then using anything that accomplishes that same purpose (cleaning the mouth) is fine and fulfills the Sunnah. *And Allah knows best.*