"The West" as a culture or civilization historically evolved out of Greco-Romanclassical antiquity. These cultures had polytheistic religions, viz. Greek polytheism and Roman polytheism. "Eastern" influences on these religions are evident from the earliest times, the Orientalizing period at the very beginning of Greek antiquity. During Hellenism and the Roman Empire period, "Eastern" religions exerted a considerable influence on "Western" religion, giving rise to Persian influenced traditions like Gnosticism and Mithraism, as well as Egyptian and "Chaldean" influence on mystery religions, astrology and magic. Early Christianity itself is a further example of Orientalizing influence on the later Roman Empire. During the same period, inherited traditions of native Roman religion were marginalized or overlaid by interpretatio graeca, and the Roman imperial cult evolved into a civil religion which involved state ritual rather than religious faith or experience. Celtic and Germanic religion was described by Roman ethnography as primitive, but at the same time as pure or unspoiled compared to the so-called urban decadence of Rome.
The principle of religious freedom introduced in Western society in the early 19th century facilitated the emergence of various new religious movements. First examples were derived from western occultism and the tradition of secret societies such as the Freemasons, but from the later 19th century, the influence of Eastern religions, notably Buddhism and Hinduism played an increasing role. From the mid 20th century, Eastern and Western spiritual traditions were increasingly syncretized in the various movements associated with the New Age and Neopagan countercultures.
The Western world, taken as consisting of Europe, the Americas, Australia-New Zealand and South Africa and Philippines, remains predominantly Western Christian: 77.4% in North America, 90% in Latin America, close to 76.2% in Europe,, 61.1% in Australia-New Zealand, 79% in South Africa and 90% in the Philippines. The second largest religions in all these regions are smaller by at least an order of magnitude, Islam in Europe with about 4%, Islam in Canada with about 3%, Judaism in the United States with about 1.7%, and Islam in Australia with about 1.7%. Most non-Christians in the Western world are irreligious, 22% in Australia, 40% in New Zealand, 18.2% in Europe, 16.4% in the USA and 16% in Canada,. This is a reflection of the tradition of secular humanism which culminated in the 18th century Age of Enlightenment. Throughout the Western world paganism is becoming increasingly popular. There remains a minority of the order of 5% of the population in the Western world which adheres to non-Western religions, mostly due to recent immigration, but to some extent also due to proselytization, notably conversion to various sects of Buddhism and Hinduism in the context of the New Age movement in the later part of the 20th century.