This is a rich and layered topic — spanning millennia, continents, and every major tradition. Let me lay it out as a coherent narrative with a clear timeline.

⚖️ The Idea of Separating “Church & State” — History, Perspectives & Ongoing Tensions

🕰️ THE TIMELINE

📌 Ancient World (pre-500 CE) — No separation; ruler = priest
In ancient Egypt, Mesopotamia, Rome, and Persia, the king was the divine authority or served at divine mandate. Caesar was a god. Pharaoh was a god. There was no conceptual divide.
Even Israel’s monarchy began as a theocracy — God was King, and human kingship was itself seen as a compromise (1 Samuel 8). The prophets held kings accountable to divine law, creating an early moral tension between political and religious authority.

📌 Early Christianity (1st–4th century CE) — Separation by necessity
Christianity’s early centuries were spent separate from and often oppressed by the state — a stark contrast to Islam, which from the lifetime of its founder was inherently connected to the exercise of state power. 
Jesus’ famous words — “Render unto Caesar what is Caesar’s, and unto God what is God’s” — planted an early seed of dual loyalty, though interpreters have debated its scope ever since.

📌 313–1500 CE — Christendom: Church becomes the State
When Constantine legalized Christianity (313 CE) and Theodosius made it Rome’s official religion (380 CE), the Church absorbed imperial power. The Pope could crown or dethrone kings. Medieval Europe became Christendom — a fusion of spiritual and political authority.
Over centuries, a common religion became both a political and spiritual mechanism connecting kingdoms to each other — consolidating power across Europe and beyond. 

📌 1517–1648 — The Reformation breaks the monopoly
Martin Luther’s rebellion shattered Christian political unity. Radical Reformers like the Anabaptists took Luther’s ideas further — arguing that the two kingdoms (sacred and worldly) should be entirely separate, and that religion should never be compelled by state power.  They approached it from the angle of protecting the church from the state.
Henry VIII, rejected by Rome, declared himself head of both the English state and the Church — ruling “by the grace of God alone.” By the early 1600s, it was well accepted in England that the government carried out the will of God through the king and his church. 
The devastating Thirty Years’ War (1618–1648), fought largely over religious lines, exhausted Europe and seeded the idea that state and religion must be separated for the sake of peace.

📌 1636–1802 — The Enlightenment and American experiment
Roger Williams in 1636 first articulated the metaphor of “a high wall” between church and state — not to attack religion, but to keep the corruption of government out of religious conscience. He called it “soul freedom.” 
Thomas Jefferson formalized the phrase “separation of church and state” in an 1802 letter, writing that religion was “a matter which lies solely between Man & his God” and that government should have no influence over it. 
Interestingly, many Protestant Americans supported separation precisely because they were Christian — fearing that state-church alliances would give one sect unfair privileges over others and destroy the personal relationship with God that Protestant faith demanded. 

📌 1789 — The French Revolution: Laïcité
France went further — not just separating church from state but actively diminishing the Church’s public role. French laïcité became the most radical version: strict exclusion of religion from all public institutions. This became the European secular model.

📌 1947 onwards — Post-colonial Muslim world’s great debate
Sayyid Mawdudi advanced the concept of Hakimiyya (divine sovereignty), arguing that secular nationalism fractures Muslim unity and elevates human authority over divine will. Sayyid Qutb similarly interpreted the Quran to mean that Islam is not merely personal belief but a complete system governing politics and society. 
Yet a significant scholarly counter-argument holds that religious and state institutions in Muslim societies were effectively separated since the death of the Prophet Muhammad — and that modern Islamist demands for a fused Islamic state reflect postcolonial discourse, not enduring Islamic precedent. 

🌍 HOW MAJOR FAITHS VIEW IT TODAY Faith General Position Christianity Deeply divided — Catholic conservatives resist full secularism; Protestant majorities often support it Islam Divided — Islamists reject it; many scholars accept pluralist governance rooted in maqasid al-Shariah (objectives of divine law) Judaism Strongly supportive — shaped by centuries of persecution as a minority; Jews thrived precisely because of the First Amendment’s church-state wall Hinduism India’s model of “positive secularism” separates personal faith from state governance while allowing the state to intervene in religious institutions for public welfare and equality Buddhism Generally supports non-theocratic governance; the Sangha (monastic community) has historically advised rulers but rarely controlled them

❓ WHY IS IT STILL OPPOSED?
Three major clusters of opposition persist:

1. Theological wholeness
   The fiercest opposition comes from deeply held theological convictions asserting that faith is not merely personal piety but a comprehensive model of government, jurisprudence, and social organization. For these voices, secular separation is not neutral — it is itself an ideology that displaces God.
2. Moral vacuum fear
   Many believers across traditions argue that when the state divorces itself from transcendent morality, it becomes morally rudderless — producing precisely the corruption and chaos that religion was meant to prevent. They see secular states as producing materialism, injustice, and inequality.
3. Minority protection (the opposite concern)
   Some argue that a strong church-state wall creates hostility to religion rather than neutrality — harming religious institutions and people of faith who wish to bring their values into public life.

🌟 The Quranic Lens
From an Islamic ethical framework, the debate isn’t really about whether divine values should inform governance — the Quran clearly makes rulers accountable to divine justice (’adl), honesty (sidq), and the welfare of the weak (masakeen). The question is whether any human institution can legitimately claim to be that divine authority — which the Quran itself guards against, given the recurring theme of Pharaonic arrogance (istikbar) in rulers who conflate their power with God’s.
The tension is ancient, universal, and unresolved — because it touches the deepest question: who has the right to govern human beings, and by whose authority?