Why secularism has Christian origins
Secularism is not a concept that other religions or civilisations developed independently.
The notion of the secular as a public realm not structured by religion is a product of European Christian civilisation.
Islam did not develop it, as sovereignty is held by Allah and Sharia represents the rules of Allah found by fiqh (Islamic jurisprudence) and grounded in revelation. The religious scholars, the ulema, skilled in fiqh, determined the law via religious rulings (fatwa). Rulers, as Muslims, submitted to the sovereignty of Allah. They appointed qadis (judges) who applied the rulings of the religious scholars. Rulers could only legitimately issue laws in the silences of Sharia — essentially, administrative law. Such decrees were their kanun.
“Hindu” civilisation did not develop such a concept, as its legal system was also based on revelation. Law grounded in revelation was adjudicated and administered by Brahmins.
Other societies did not develop it, as ritualisation emanated out from the rituals of hearth, home and clan in a common epistemic realm. Including the hearth, home and clan of the ruler. There was no law-giving religious authorities nor a concept of separate epistemic realms.
Christianity is a scriptural and congregational religion. With a very clear notion of religious authority. From Late Antiquity onwards, Christianity developed a highly structured religious hierarchy. But the authority of customary law, or the capacity of rulers, or other bodies, to issue law was not challenged even when Christian doctrine was invoked to change or direct the content of such laws. Even canon law, however much it might reference revelation, was not directly based on revelation, but was issued by a human institution (the Church) and subject to revision by it.
Christianity grew up in the law-ruled Roman Empire: it did not need to develop its own structure of laws. Muhammad founded a conquest state, hence Islam not only has an inbuilt legal structure, it has an inbuilt imperial legal structure. Sharia had no pre-imperial existence.
In the medieval disputes between Pope and Emperor, such as the Investiture Controversy, there was a very clear sense that both had authority, the question was the boundaries and relations between them.
Once the denominational unity of Latin Christendom was shattered by the Reformation, people acknowledging different religious authorities had to be grappled with. The destructive nature of the European Wars of Religion led to the explicitly secular project of naturalising political theory, of changing the grounding question of political thought from the relationship of God, man and society to the relationship of man, society and the world. Changing the key question was the project of Hobbes, Locke, Rousseau, and other Enlightenment thinkers.
This Enlightenment shift was to take an existing distinction within Christian thought and society and develop it further. But it was a shift that built on explicitly, and distinctively, Christian roots.
Moreover, it was a shift entirely internal to the dynamics of European societies. Thus, the Polish-Lithuanian Commonwealth granted freedom of religion in 1573. Even the new Constitution of the United States barring the federal government from establishing any religion was reacting to European legacies and dilemmas in the context of federating colonies with different established religions.
In the settler colonies, the European settlers brought their religion with them. Indeed, protecting and fostering their religious identity was frequently a motivation for such settlement. In the non-settler colonies, the European administration was such a thin layer, that local religions were largely left alone, except as required to maintain public order. The British campaigns against thuggee and of various colonial powers in India against suttee were very much about maintaining a certain level of public order.
Missionary activity was sometimes fostered, sometimes hindered, but never a prime concern of the colonial administrations. Though it was occasionally an excuse for imperial expansion. Religious norms certainly entered into imperial law, but as a reflection of the norms in their domestic laws. Imperial laws tended to be slightly more secular than the laws “back home” precisely because of the religious diversity and differring religious patterns of the societies they administered compared to what was the case “back home”.
The constraints of imperial administration interacted with the dynamics generated from within European societies. But that was the general pattern of European imperialism.