The "Shari`a" or "Applied Islamic Law inferred from Revelation" knows only two credal categories of human beings: "Muslim" and "non-Muslim" (kâfir). There is no difference of opinion on this in Islamic Law, heresiology, and doctrine. There is a difference of opinion whether the "people of the period in-between Prophets" (Ahl al-Fatrah) fit in the first or the second category.
Within the "Muslim" category there are the "righteous" and the "unrighteous," the "heretics" and the "orthodox," etc. I use "heresy" and "orthodoxy" in the "Sunni" sense. Within the "non-Muslim" category there are the "People of the Book": "Jews," "Christians," and "Zoroastrians"; the "people of idols" and "polytheists" (mushrikûn); the "atheists" (mulhidûn); the "Satanists," etc.
If a "Hindu" or a "Buddhist" or a "Stoic" or a "Mushrik" is very devout and does great service to mankind and is a great asset to country and the whole world, indeed if such a person saved the life of the Prophet Muhammad himself (upon him blessings and peace) and his immediate Family, his relatives at large, and the totality of his followers at the time, he remains a "Kaafir" in terms of the "Shari`a."
However, the Law steers clear of any other-worldly judgment concerning such a person and does not categorically forbid speculation that Buddha or the like might have been God- sent Prophets or that there are elements of Abrahamic monotheism in non-Abrahamic creeds and philosophies.
Those that focus on such an other-worldly judgment might countenance the salvation of even Pharaoh or Satan. The Shari`a does not consider such a scenario a categorical impossibility, although it is a categorical impossibility that Allah be unjust.
The positions highlighted below are "heresy" for sure and possibly "kufr":
Ijma [of the Sahaba] ... can, therefore, not be considered [a] primary source... of Islam. The Qur'an makes it categorically obligatory to follow "the path of the Believers," which makes Ijma` of the Sahaba binding.
Does he or does he not believe there is only One True God? That is the difference between believer and unbeliever. Period. Then Satan would be a believer. No: you must believe in all the Prophets as well.
The Quran IS the main and only absolutely authoritative source of Islam. Everything else is opinion. The Qur'an itself says the Prophet does not speak from whim. To say that such as the Prophet's statement "Nothing but truth comes from this mouth" is "opinion," is undoubtedly "kufr".
A Muslim by name can be a kaffir as much as a Hindu. No, the Law deems a "Muslim" a "Muslim" until he unambiguously denies an obligatory part of the creed, at which time he ceases to be a "Muslim." The Law is not a mind reader.