In any case I think NFAK's genius transcends his faith. His gift is not about Islam, to me. It's about something much bigger.
I was just listening to a youtube video and saw that it had subtitles. One of the subtitles moved and surprised me. "Why do Hindus and Muslims fight?" The audience made approving noises.
I re-watched the video and paid more attention to the subtitles. NFAK is expressing ideas straight out of the psalms: God is omnipresent; all creation is witness to God; creation yearns for God: Psalm 19, Psalm 139, Psalm 42.
I googled "Nusrat Fateh Ali Khan religious beliefs" and found a page that quotes the lyrics of one of his songs. I find these lyrics immensely moving. They speak of one finding God in a temple, another in a mosque:
"One lover was in the temple and another in the mosque
but to me, immersed in the joy of love, both seemed same
Chanting on rosary, the name of Shyam [Lord Krishna], I become him.
I am worthless except that
I surrender to the name of my beloved, all the time." (source)