the fusion of diverse religious beliefs and practices. Instances of religious syncretism—as, for example, Gnosticism (a religious dualistic system that incorporated elements from the Oriental mystery religions), Judaism, Christianity, and Greek religious philosophical concepts—were particularly prevalent during the Hellenistic period (c. 300 BC–c. AD 300). The fusion of cultures that was effected by the conquest of Alexander the Great (4th century BC), his successors, and the Roman Empire tended to bring together a variety of religious and philosophical views that resulted in a strong tendency toward religious syncretism. Orthodox Christianity, although influenced by other religions, generally looked negatively upon these syncretistic movements.

Syncretistic movements in the Orient, such as Manichaeism (a dualistic religion founded by the 3rd-century-AD Iranian prophet Mani, who combined elements of Christianity, Zoroastrianism, and Buddhism) and Sikhism (a religion founded by the 15th–16th-century Indian reformer Guru Nānak, who combined elements of Islām and Hinduism), also met with resistance from the prevailing religions of their respective areas.

In the 17th century a movement led by the German Protestant theologian George Calixtus aimed at reconciling the differences between the Protestants in Germany, but his efforts were disparaged by orthodox Christian leaders as syncretistic.