St. Thomas Aquinas believed that the state, like the church, was a complete community, both of which, together with the larger community, should function to promote the common good. Aquinas believed that the state was subject to moral laws and justice. For Aquinas, there is a difference between the secular state and the Church, but this distinction is not always clear.
Aquinas theorized about a government mixed with monarchy, aristocracy, and democracy; a monarch and a cabinet whom should be chosen democratically. No one, even the monarch, is above the law unless it serves the moral/common good. If a ruler becomes a dictator, the people could depose him.
The state should defend its people from criminals and potential enemies in the case of invasion or war. The state's role is to suppress any external factors that might infringe upon a citizen's freedom or will. The state should suppress anything that threatens the common good of the state. Aquinas states:
For human law's purpose is the temporal tranquility of the state, a purpose which the law attains by coercively prohibiting external acts to the extent that those are evils which can disturb the state's peaceful condition.
In other words, the state prohibits any outward behavior that threatens the peaceful state of the community.
Even if the government is run by a king with absolute power, as opposed to a ruler or group with limited powers, the people have an assumed covenant with their rulers that if that ruler/rulers violate laws and/or the peace of the state, the people have the right to resist this government.
Although political authority is derived from divine morals, the state is not a divine authority. The state guides its citizens, through enforcement of laws, to be virtuous but the state can only instruct external behavior. Internal or mental instruction is more under the guidance of the Church, spiritual revelation, and God. This is one of the main differences between the two authoritative communities (the State instructs natural, outward behavior and the Church instructs spiritual knowledge). The Church has no jurisdiction over the State and the State has no direction over the Church unless the peace of the community is threatened. Although this seems like an early version of "the separation of church and state, Aquinas is less clear about how much influence the Church has on the State and vice versa.