In the Middle Ages, the Church and Christian monarchies inherited a political model from the Roman Empire, known as the theologico-political system —a system in which power is considered sacred, meaning that the political leader is also a religious leader.

This is why a politico-religious unanimity characterizes medieval societies. Political power bases its legitimacy, authority, and unity on the Christian faith (or the Muslim faith). It considers itself the guardian of cultural and religious orthodoxy and treats as pariahs those who stray from this unanimity. In this context, even if a certain tolerance could be conceded to those who detach from the common cultural vision (such as Jews), no right to pluralism could be recognized for them. It was not until the end of the Middle Ages, with the conquest of America, that the issue of civil liberties became crucial to the Church, giving rise to a new philosophy of law that affirmed and protected individual freedoms, legitimized pluralism, and condemned state coercion.

The question of the relationship between politics and religion took shape with Saint Augustine's work Civitas Dei (The City of God). In it, he explains that two spheres coexist: Two loves have thus made two cities: the love of self to the contempt of God, the earthly city; the love of God, to the contempt of self, the heavenly city.

We have, therefore:

However, for Augustine, this earthly city is necessary. It is essential because it guarantees peace. Thus, coexistence with the religious must be well managed, and a predominance of spiritual power over temporal power should regulate it. However, there should not be a radical separation or open conflict; instead, both entities should work together. Historians have referred to this doctrine as political Augustinianism.

However, neither popes nor kings were satisfied with this alliance. The Church attempted to assert its authority over political power, while political power sought to free itself to assert its sovereignty.

Thus, the Church, on its part, will develop its law and courts and will assert that the Pope can settle earthly disputes. On their side, kings will begin to establish a state apparatus as powerful as possible. They will also attempt to centralize the resolution of legal conflicts, gradually generalize taxation, develop territorial administration, and raise armies, thereby laying the groundwork for the modern State.

In reality, the competition between the powers led to numerous conflicts. Each Prince or pope always tried to have the last word and convince others that he held the supreme authority, as a last resort. Thus, Pope Gregory VII declared:

On his side, Saint Louis did not hesitate to oppose Pope Innocent IV, who had excommunicated and deposed Emperor Frederick II, thus depriving him of all credibility among his people. His grandson, Philip the Fair, would do the same.

The Church's theocratic temptation also clashes with the theory of "divine right." If the kings of France proclaim themselves monarchs by "divine right," it is to escape the Pope's grip and draw their authority directly from God, without having to receive orders from the clergy.