Syncretism (/ˈsɪŋkrətɪzəm, ˈsɪn-/) is the practice of combining different beliefs and various schools of thought. Syncretism is when certain elements, features and components of a religion are incorporated and absorbed into another religion or faith. It directs some sort of difference in both religions. Syncretism involves the merging or assimilation of several originally discrete traditions, especially in the theology and mythology of religion, thus asserting an underlying unity and allowing for an inclusive approach to other faiths. While syncretism in art and culture is sometimes likened to eclecticism, in the realm of religion, it specifically denotes a more integrated merging of beliefs into a unified system, distinct from eclecticism, which implies a selective adoption of elements from different traditions without necessarily blending them into a new, cohesive belief system.

The English word is first attested in the early 17th century. It is from Modern Latin syncretismus, drawing on the συγκρητισμός (synkretismos), supposedly meaning "Cretan federation". However, this is a spurious etymology derived from the naive idea in Plutarch's 1st-century AD essay on "Fraternal Love (Peri Philadelphias)" in his collection Moralia. He cites the example of the Cretans, who compromised and reconciled their differences and came together in alliance when faced with external dangers. "And that is their so-called Syncretism [Union of Cretans]." A more likely etymology is from sun- ("with") plus kerannumi ("mix") and its related noun, krasis ("mixture").

Religious syncretism is the blending of two or more religious belief systems into a new system, or the incorporation into a religious tradition of beliefs from unrelated traditions. This can occur for many reasons, and the latter scenario happens quite commonly in areas where multiple religious traditions exist in proximity and function actively in a culture, or when a culture is conquered, and the conquerors bring their religious beliefs with them, but do not succeed in entirely eradicating the old beliefs or (especially) practices.

Religions may have syncretic elements to their beliefs or history, but adherents of so-labelled systems often frown on applying the label, especially adherents who belong to "revealed" religious systems, such as the Abrahamic religions, or any system that exhibits an exclusivist approach. Such adherents sometimes see syncretism as a betrayal of their pure truth. By this reasoning, adding an incompatible belief corrupts the original religion, rendering it no longer true. Indeed, critics of a syncretistic trend may use the word or its variants as a disparaging epithet, as a charge implying that those who seek to incorporate a new view, belief, or practice into a religious system pervert the original faith. Non-exclusivist systems of belief, on the other hand, may feel quite free to incorporate other traditions into their own. Keith Ferdinando notes that the term "syncretism" is an elusive one, and can refer to substitution or modification of the central elements of a religion by beliefs or practices introduced from elsewhere. The consequence under such a definition, according to Ferdinando, can lead to a fatal "compromise" of the original religion's "integrity".

In modern secular society, religious innovators sometimes construct new faiths or key tenets syncretically, with the added benefit or aim of reducing inter-religious discord. Such chapters often have a side-effect of arousing jealousy and suspicion among authorities and ardent adherents of the pre-existing religion. Such religions tend to inherently appeal to an inclusive, diverse audience. Sometimes the state itself sponsored such new movements, such as the Living Church founded in Soviet Russia and the German Evangelical Church in Nazi Germany, chiefly to stem all outside influences.

Véronique Altglas argues that the use of the term "syncretism" is an effort to make some religious practices and religions seem less legitimate, pure, or well-established. She argues that the term "syncretism" presupposes that the religions that syncretism combines were "pure" and unchanging before being combined. In reality, she believes, religions are all inherently syncretic to some degree because they combine many elements from different people's spiritualities and religions and different cultures. The term "syncretism" can serve the ends of ethnocentrism and purism in some cases in this view.

According to some authors, "Syncretism is often used to describe the product of the large-scale imposition of one alien culture, religion, or body of practices over another that is already present." Others such as Jerry H. Bentley, however, have argued that syncretism has also helped to create cultural compromise. It provides an opportunity to bring beliefs, values, and customs from one cultural tradition into contact with, and to engage different cultural traditions. Such a migration of ideas is generally successful only when there is a resonance between both traditions. While, as Bentley has argued, there are numerous cases where expansive traditions have won popular support in foreign lands, this is not always so.

In the 16th century, the Mughal emperor Akbar proposed a new religion called the Din-i Ilahi ("Divine Faith"). Sources disagree with respect to whether it was one of many Sufi orders or merged some of the elements of the various religions of his empire. Din-i Ilahi drew elements primarily from Islam and Hinduism but also from Christianity, Jainism, and Zoroastrianism. More resembling a personality cult than a religion, it had no sacred scriptures, no priestly hierarchy, and fewer than 20 disciples, all hand-picked by Akbar himself. It is also accepted that the policy of sulh-i-kul, which formed the essence of the Dīn-i Ilāhī, was adopted by Akbar as a part of general imperial administrative policy. Sulh-i-kul means "universal peace".