How did it spread across cultures and regions?

By preaching, trade, and just rule — not forced conversion, which the Qur'an forbids (Qur'an 2:256). Today it is the fastest-growing religion, largely by people choosing it.

Through books, universities, and science education; through the broad secularization of Europe and East Asia; through state promotion in the Soviet Union and China; and, in this century, through the internet, which spread its arguments among younger generations worldwide.

Less a movement than a stance: it spread with modern education, science, and religious pluralism, and it is most common wherever people know several faiths well and hesitate to judge between them.

Along Roman roads and in the Greek language across the empire; made the Roman state religion in 380 CE; carried through Europe by missions; split East and West (1054); then carried worldwide by European colonial expansion and missionary movements — its fastest growth today is in Africa and the Global South.

Mainly by diaspora rather than mission: communities in Babylonia and around the Mediterranean, later the Ashkenazi and Sephardi centers of Europe and the Islamic world. Expulsions and migrations kept moving its centers; the founding of Israel (1948) regathered roughly half of world Jewry.

South to Sri Lanka and Southeast Asia (the Theravada school); along the Silk Road to China, Korea, and Japan (Mahayana); and to Tibet (Vajrayana) — carried by monks, merchants, and royal patronage. It had nearly vanished from its Indian homeland by the medieval period.

Mostly within the Indian subcontinent. In the first millennium CE it reached Southeast Asia (Angkor; the island of Bali, where it survives today); in the modern era, migration carried it worldwide, and 19th-century reform movements presented it to global audiences.

It stayed rooted in Punjab and spread mainly through Punjabi migration — to East Africa, Britain, North America, and Southeast Asia — rather than through missionary campaigns.

Through constitutions, courts, and public schooling rather than conversion — exported with European legal models and adopted in different forms: American accommodation, French strictness (laïcité), Indian 'principled distance,' Turkish laiklik.

Through ethical societies, humanist associations, education, and above all the language of human rights — the Universal Declaration (1948) speaks its vocabulary. It is strongest in Europe and the English-speaking world.

Through revolutions, constitutions, and trade; ambiguously through empire — liberal powers preached liberty while ruling illiberal empires; and after 1945 through international institutions and human-rights law. After 1989 it briefly seemed to have no rival.

In waves: suffrage internationally (the first); workplace and legal equality in the 1960s–70s (the second); diversity and intersectionality (the third); and online activism such as #MeToo (the fourth) — carried by movements, legal reform, and UN instruments like CEDAW (1979).

Through the 19th-century unifications (Italy, Germany); the collapse of empires after 1918 under the banner of self-determination; and decolonization after 1945, when anti-colonial movements everywhere adopted it. Today nearly every state on earth presents itself as a nation-state.