I understand that Ayah Tathir establishes the infallibility of the Ahl al-Kisa (AS), but where in the Quran does it say that the other Imams (AS) are infallible as well? If this belief originates from Hadith then where can the evidence be found?
The five Ineffable greatest personalities (The Prophet (SAWA), Imam Ali (AS), Fatima (AS), Imam Hasan (AS) and Imam Husain (AS) have repeatedly said in many authentic Hadeeths that all the nine following Imams are Infallible. Believing in Hadeeth al-Kisa leads definitely to believing that what ever was said by the greatest Five is the absolute truth which is strong evidence that all the twelve Imams are Infallible.
Wassalam.