SOMMARIO: 1. Introduction – 2. Part I: Theological and Historical Roots of Western Constitutionalism – 2.1. King-Servant of God vs. Tyrant-Servant of the Devil – 2.2. Hobbes and la religion de l’État – 2.3. John Locke’s Principle of Tolerance. The Modern Constitutionalism – 2.4. Roger Williams: The Apostle of Religious Liberty. The Roots of American Secularism – 3. Part II: The secularismlaïcité – 3.1. The Double Separation (Emancipation): State from Religion, Religion from the State – 3.2. Islam and la laïcité à la française – 3.3. Individual Rights vs Group Rights – 4. Conclusion. Secularism and Supranational Constitutionalism