What is the ruling on one who follows a heretical school of thought, and what is the evidence for that?

The questioner must follow the methodology of Ahlus-Sunnah wal-Jama'ah, for it is the true path, and its foundation is the Book and the Sunnah. This can be recognized by ten qualities mentioned by Sahl ibn Abdullah al-Tustari, among them: not abandoning the congregation, not reviling the Companions, not revolting against the Ummah with the sword, not denying divine decree (Qadar), and not doubting faith.

Every group that claims to follow Islam has its rightness and wrongness measured by its proximity to or distance from the methodology of Ahlus-Sunnah wal-Jama'ah. Among them are groups with innovations that lead to disbelief, and some with innovations that do not lead to disbelief.