The way to debate with Takfiris
In the year 38 AH, Imam Ali (A.S) sent Abdullah ibn Abbas for talks with Khawarej and in a letter told him: Do not debate with Khawarej with Qur’an because the holy Qur’an has general views which may have various interpretations, you say one thing and they say another. But debate with them with the tradition (Sunnah) of the Prophet (PBUH) where they will have no choice but to accept [1].
[1] Nahjul Balagha, Letter Number 77.
Other links:
Imam Ali (A.S) The master of successors
The Greatness of the Creator
Backbiting
