Not most of Sunni Muslims but only the followers of Mu'awiya and Yazeed claim that Abu Talib was non Muslim. Many Sunni scholars have written books proofing the Faith of Abu Talib as a great believer in Islam like Ahmad Zaini Dahlan the Mufti of Makkah in his book (Asna al-Mataalib ) al-Barzanchi in Bughyat al-Talib, and al-Suyooti al-Shafi'ee in Bughyat al-Talib, and Muhammad Mu'een al-Hanafi al-Sindhi in his book ( Ithbaat Islam abi Talib) and ibn Abi Sayf al- mada'ini in his book (Akhbaar Abi Talib Wa Wildih) other scholars.
'The narrations claimed against Abu Talib are fabricated to please the anti Shia rulers. The scholars in Knowledge of Hadeeth have strong evidence against such narrations.
The Proohet Muhammad (SAWA) did not allow any Muslim lady to remain with her non Muslim husband including asking his own fostered daughter to leave her non Muslim husband, but he never asked Fatima Bint Asad , the great lady of faith who was called by him as (My mother) to leave her husband Abu Talib till the end of his life. This is itself is a concrete evidence that Abu Talib was a believer in Islam.