What is the basis upon which those who say that Islam is divided into 72 sects, one of which is correct and the rest are in Hellfire, determine this one sect?

The basis for the division of the Ummah is the Prophet's (peace be upon him) hadith: "The Children of Israel split into seventy-one sects, and my Ummah will split into seventy-two sects, all of which will be in the Fire except one, which is the Jama'ah (the main body of Muslims)." The blameworthy division is not in the subsidiary matters of fiqh (jurisprudence), but rather in the fundamentals of Tawhid (monotheism), the assessment of good and evil, the conditions of prophethood, and loyalty to the Companions. This hadith is an announcement of what will come to pass, not a justification for division, for the Shari'ah (Divine Law) of Allah is one and without divergence.