Syncretism is the blending of elements from different religious traditions into a new configuration. The phenomenon is close to universal; the term is deeply contested, and the contest is largely about who is entitled to apply it to whom.
The word's history explains the trouble. Plutarch used it of Cretan communities uniting against a common enemy, but its modern career begins in seventeenth-century Protestant theological polemic, where "syncretistic" was an accusation levelled at attempts to reconcile Lutheran and Reformed positions. It has carried a pejorative charge ever since, and it has been applied overwhelmingly by observers to other people's religion, and particularly by Europeans to the religions of the colonised. Very little religion is ever described as syncretistic by those who practise it.
The critical objection follows directly from that history. If every tradition is in fact a product of contact, borrowing and adaptation, and every one demonstrably is, then calling some of them syncretistic implies that others are pure, which no historical account supports. Christianity took its philosophical vocabulary from Greek thought, its legal categories from Rome, and a good deal of its calendar from local practice. Buddhism absorbed local deities and cosmologies in every country it entered. Islam incorporated existing practice at Mecca and far beyond it. The label therefore frequently functions as a way of marking a tradition as derivative and inauthentic while exempting the speaker's own from the same analysis.
Scholarship since the 1990s, particularly the volume edited by Charles Stewart and Rosalind Shaw, has responded by studying the politics of the term rather than simply applying it, asking who calls what syncretic, against whom, and to what end. The corresponding term "anti-syncretism" names the policing of boundaries by those claiming authenticity, which is itself a common and consequential religious activity.
The cases usually cited remain instructive provided the framing is handled with care. Afro-Caribbean and Afro-Brazilian traditions in which orisha are associated with Catholic saints, an association whose origin under conditions of enslavement and prohibition is well documented, and whose present interpretation varies among practitioners, some treating it as a historical disguise now discardable and others as a genuine identification. Latin American Catholicism incorporating indigenous practice. Chinese religion, in which Confucian, Daoist and Buddhist elements combine without any expectation of exclusivity, and where the assumption that one must choose a single religion is itself a foreign imposition. Japanese practice combining Shinto and Buddhism. And Sikhism, sometimes described as a synthesis of Hindu and Muslim elements, a characterisation Sikhs generally reject as denying the tradition's own account of an independent revelation, and which scholars increasingly avoid for that reason.
That last case makes the general point. Whether a tradition is a mixture or an original is a question with real consequences for its standing, and the term is very rarely neutral.