Who helped Islaam more than Abu Taalib, the uncle of Rasoolullaah صلى الله عليه وسلم? Read up on the Seerah. There's never been a person - from that time till the present day - who helped Islaam more than Abu Taalib did. Abu Taalib defended Rasoolullaah صلى الله عليه وسلم with his life and his wealth. He helped to protect Islaam. Yet, Abu Taalib died as a Kaafir, and when Rasoolullaah صلى الله عليه وسلم wanted to make Du`aa of Maghfirah (Forgiveness) for him, he was prevented by Allaah Ta`aalaa, and Allaah Ta`aalaa revealed the Aayah:
إنك لا تهدي من أحببت ولكن الله يهدي من يشاء
"You cannot guide those whom you love, but it is Allaah who guides whoever He wishes..."
And Rasoolullaah صلى الله عليه وسلم informed us that his own uncle, Abu Taalib, who helped Islaam so much, will be in Jahannam, and that he will have the lightest punishment in Jahannam. He will be made to wear shoes of fire which will cause his head to boil.
Who is Mandela and Tutu, in comparison to Abu Taalib...? If even Abu Taalib will be in Jahannam, because he did not die on Islaam, then who are they or anyone else...?
Also, we do not know what is in the hearts of people. If a person says that he is a Muslim, we regard him as a Muslim (provided he does not commit one of the things which nullifies Imaan). Mandela and Tutu never in their lives Ever claimed to be Muslims. If a person believes in Allaah Ta`aalaa but does not believe in Rasoolullaah صلى الله عليه وسلم, then he is a Kaafir. The Shahaadah consists of two parts:
"I testify that there is no Ilaah (God) except Allaah, and I testify that Muhammad (صلى الله عليه وسلم) is His slave and messenger."
The person has to become Muslim in THIS Dunyaa. Every single person will believe in Allaah Ta`aalaa in the Aakhirah, obviously. Their Imaan will not help them then. They have to become Muslims here, while they are still alive. There is no "helping them to recite the Kalimah" after they have died. A Kaafir who has died can recite the Kalimah a billion times, it will not help him. He has to recite it before he leaves this Dunyaa. While he is still living here.
They were not Muslims. They never accepted Islaam. That is the end of the matter.
There is no Jannah for a person who dies upon Kufr, even if he or she had done the world of good.
Was-Salaam.