“If you divorce politics from religion where do political institutions draw their legitimacy to govern?” I was just trying to give voice to the question that might have had perturbed the early modern thinkers.
Early modernity was a period of turmoil. The English civil war that lasted for nine years was instigated by the oppressive rule of Charles I of England. He ruled under the principle of divine right of the king and subjected therefore the people into onerous taxation, illegal arrest, land grabbing and many despotic acts. His father before him, James I, the king who sponsored the production of King James Version of the Bible, openly claimed on his speeches and writings that he was divinely appointed as king of England. Earlier, Henry VIII severed his ties with the Roman Catholic Church and declared the Church of England independent and made himself as its Supreme Governor. Having reprimanded, however, that the Bible remained higher than his office, he immediately declared that the king is the only authorize person to interpret it. Making King Henry VIII automatically higher than the Bible. In France, on the other hand, King Louis XIV, probably well acquainted with archaic religions, claimed to be the sun-god himself.
Early modern thinkers were problematizing this marriage of religion and politics, which, more often than not, resulted into abusive form of regime. Sacred kingship, however, has a long history; it is embedded in our culture and consciousness. In one stele found in Egypt, you can see Ramses II standing between the gods Horus and Khnum, which strongly suggests the ancient belief of the Egyptians to divine kingship. The throne of David was also celebrated in Israel as the throne of Yahweh. In critical accommodation, Bishop Eusebius of Caesarea, accorded to Emperor Constantine that sacred kingship which basically started the revival of divine kingship in the West. With this, early modern philosophers were faced with the challenge to remove politics from its ugly marriage with religion. That begs the question, where are you going to draw its legitimacy if politics will refuse the legitimacy provided by religion.
To cut short the discussion, such legitimacy was provided by the people. From divine covenant, political institutions turned into social contract, from divine laws they turned into constitution which was made and ratified by the people, and from divine sovereignty they turned to the people as the sovereign power who legitimizes political institutions. The early modern thinkers from Baruch Spinoza to Thomas Hobbes to John Locke worked and proposed its insipient version, and eventually developed into its mature version later. Legitimacy, power and authority no longer emanated from above, but from the people themselves. That is the spirit of democracy; that is the spirit of constitutional government!
In saying this, the success and failure of a democratic state depends on two things: 1) how the collective dreams of the sovereign citizens is being pursued and protected by the state, 2) and how the laws of the land are being uphold, respected and implemented fairly and justly. Any violation to these two goals or principles would result to political and constitutional crisis which automatically strip off the state of its legitimacy to rule. A ruler who is technically illegitimate because of his abandonment of his political and constitutional mandates would expected to course to violence to keep his position. And that does not only disqualifies him but render him unfit and anathema to a democratic rule.