Here’s the summary from Jacques Rousseau.
We do now know that Mandela was a member of the Communist Party, and some might therefore think it follows that he was an atheist. On the other hand, we do know that he was baptised as a Methodist, and we have Wikipedia quoting an interview with Mcebisi Skwatsha, in which Mandela apparently confirmed that he was a Methodist. In Mandela’s book Conversations with Myself, he says ”I never abandoned my Christian beliefs”, and a comment on Pharyngula points to a CNN story, where it’s described how Mandela would regularly receive blessings from Chief Rabbi Cyril Harris.
Mandela also spoke at churches on a semi-regular basis, and in short, clearly seemed to have no antipathy to religion. Instead, his attitude to religion seems to have been exactly the right one for the leader of a nation to have – to hold it as a personal issue, and to devote himself to allowing others to exercise their religions, or lacks of religion, in the manner they see fit. In other words, regardless of what his personal beliefs were, he seems to have been fully committed to secularism in government.
He was a secularist. That ought to be good enough.