Prophet’s (s.a.w.a.) affection for Abu Talib (a.s.) is proof of his faith and Islam
The skeptics do not consider Abu Talib (a.s.) a Muslim despite his tireless service in the way of Islam and in the defence of the Holy Prophet (s.a.w.a.).
If only the skeptics had paid attention to the Holy Quran and the bond of love and affection between uncle and nephew, they would not cast doubt on the faith and Islam of Abu Talib (a.s.).
The love of the Holy Prophet (s.a.w.a.) for Abu Talib (a.s.) is well-known and well-acknowledged. Had Abu Talib (a.s.) been an infidel (Allah forbid), then loving him would not have been permissible for the Prophet (s.a.w.a.) as Allah says:
‘You shall not find a people who believe in Allah and the latter day befriending those who act in opposition to Allah and His Apostle, even though they were their (own) fathers, or their sons, or their brothers, or their kinsfolk…’ (Surah Mujadilah (58): 22)
Sharh Nahj al-Balaghah v 14 p 69-70
Mirah al-Uqool v 26 p 371
Behaar al-Anwaar v 35 p 157
The skeptics can allege all they want about Abu Talib (a.s.) at the cost of rejecting the Holy Quran and / or the Sunnah of the Holy Prophet (s.a.w.a.) i.e. affection for his uncle