What is the meaning of the Hadith: "My Ummah will not unite upon error," and how do we respond to those who say that the large number of followers of the Tablighi Jamaat – despite the presence of polytheism (shirk) in some of their books – indicates that they are not in error, based on this Hadith?

The hadith "Indeed, Allah will not unite my Ummah upon error" indicates the infallibility of all scholars of the Ummah from uniting upon falsehood. The "group" (jama'ah) referred to here are the people of jurisprudence, knowledge, and hadith, not the agreement of some scholars or most of them, nor the common people. A large number of followers is not proof of truth, for Christians are greater in number, and the people of innovation may outnumber the people of Sunnah in some lands. Truth is known by its conformity to the Book, the Sunnah, and the consensus of the predecessors (Salaf), not by a large number. The people of truth are often few.