Middle and Modern period

The Middle Ages (800 AD-1500 AD) was a time of philosophical development for several major religions. As Christianity became the focus of scholarship throughout Europe, Buddhist missions were sent to East Asia, and Islam was spread throughout the Middle East, Central Asia, Southeast Asia, North Africa and parts of Europe and India. Meanwhile, the decline of Buddhism in India led to the flourishing of folk religion there.

Many medieval religious movements emphasized mysticism, such as the Cathars and related movements in the West, the Bhakti movement in India and Sufism in Islam. Monotheism was articulated distinctly in Christian Christology and in Islamic Tawhid. Hindu monotheist notions of Brahman likewise reached their classical form with the teaching of Adi Shankara.

Religion was the dominant ideology behind many conflicts of the Middle Ages. Muslims were in conflict with Zoroastrians during the Islamic conquest of Persia; Christians were in conflict with Muslims during the Byzantine-Arab Wars, Crusades, Spanish Reconquista and Ottoman wars in Europe; Christians were in conflict with Jews during the Crusades, Reconquista and Inquisition; Shamans were in conflict with Buddhists, Taoists, Muslims and Christians during the Mongol invasions; and Muslims were in conflict with Hindus and Sikhs during Muslim conquest in the Indian subcontinent.

Modern period
European colonisation during the 15th to 19th centuries resulted in the spread of Christianity to Sub-Saharan Africa, the Americas, Australia and the Philippines. The 18th century saw the beginning of secularisation in Europe, rising to notability in the wake of the French Revolution. By the 20th century, religion was no longer the dominant ideological force behind international wars, but had generally been unseated by political ideals such as democracy and communism.

In the 20th century, the regimes of Communist Eastern Europe and Communist China were explicitly anti-religious. A great variety of new religious movements originated in the 20th century, many proposing syncretism of elements of established religions. Adherence to such new movements is limited, however, remaining below 2% worldwide in the 2000s. Adherents of the classical world religions account for more than 75% of the world's population, while self-reported alliegance to indigenous folk religions has fallen to 4%. As of 2005, an estimated 14% of the world's population identifies as nonreligious.