Church-State relations have always been a bit testy. I think the apologists were mostly correct: God may have permitted states to form, "because of the hardness of our heart," but basically that power or authority is going to be used badly, as often as not. Part of the mission of the Church is to curtail that abuse, to "Christianize" the state. In a very schematic way, we can think about it in four phases:
1. Early Church, "Age of Martyrs" - characterized by a Church trying to distinguish itself clearly (in its intellectual, social, moral, and liturgical life) from everything around it (Judaism, Paganism(s), Heresies).
2. Constantinian Church - After Constantine's conversion, he intervened (or interfered) in Church matters, including disputed ownership of dioceses, public policy favoring Christians and churches (taxes, wills, etc), and theology - up to and including the First Council of Nicaea in 325. Constantine called himself "bishop for external affairs" and applied the old prerogatives of the pagan "Pontifex Maximus" to the Church. Since we're being schematic, we can say that this arrangement lasted in the East more or less until 1453.
3. Medieval Church - Pope Gelasius I offered the so-called "two-swords" theory, dividing civil and ecclesiastical power. Since we're being schematic, we can say that in the Early Middle Ages, the Church authority was independent of the state's, but generally weaker. Kings, dukes, and counts had a lot of say in Church affairs, even to choosing (or at least approving) key Church appointments. In the High and Later Middle Ages (after the Investiture Controversy), the Church's authority was independent of the state's, but generally stronger. The Church has a lot of say in political affairs, even to choosing or approving kings, marriages, and successions. Positively, it was felt that the Church, being morally free in Christ, could morally direct the use of the state's coercive power (police, law systems, justiciary); while lacking that power itself would be a check on human weakness. The state, by cooperating with the Church in this way, could be more the instrument of divine rule God had intended it to be; while not choosing the end of its own coercive power would be a check on human weakness.
4. Since the Treaty of Westphalia (1648) - The Reformation and its aftermath dramatically changed the relationship of Church and State. States in the 16th century completed the subordination of the nobility, and the extension of the powers of royal (i.e. central) government over all associated territories (formerly duchies, counties, etc). They successfully monopolized for that royal or central government the use of coercive force. The Church was now the only non-physical limit to the power of the king, or later of Parliament or Prime Minister or President. But the Church itself was both damaged and discredited in the Reformation and what followed, so it couldn't really perform that function well. The coercive power of the state was uncoupled from the Church's direction, mostly so that the competing churches couldn't use it against each other, as in the Wars of Religion and 30 Years War. The State became morally self-regulating in its use of coercion. Hence the creation of the modern "rights of man" language, that tries to set clear limits on how the state can use its unrivaled power. But since the 19th century and industrialization, that tradition has been breaking down in practice. Fascism and Communism have the same roots as we do, and in each case the State devoured itself. Liberalism (representative democracy regulating a common, relatively free market) has all the same risks.
Since the 19th century, meanwhile, the Church has been rebuilding its moral vigor. Ecumenism is overcoming the partisan divisions of the past, and the common moral vision of our Christian heritage is more easily articulated. A document like Gaudium et Spes clearly shows the desire of a united front of Christians committed to being a moral force in the world, and a check on the otherwise autonomous (unchecked) civil power which now dominates it. The pro-life battleground is the obvious place this is playing out most confrontationally. Marriage issues are quickly becoming another. Christians can - must - choose between this reinvigorated prophetic Christianity (magisterial Catholics, most Evangelicals, most Orthodox, a few other Protestants) and the limpid sold-out squooshiness of decaying hippy progressive Christianity (cafeteria Catholics, mainstream Protestants).
This makes it clear that the Church's future is with the New Evangelization. Spineless liberal Christianity, which has abandoned dogma and Tradition, rejected Revelation, and explained away every miracle of salvation history, has nothing at all with which to play its proper role in limiting the immoral abuse of the state's growing power. In accepting as "spilled milk" fornication, adultery, abortion, and homosexuality - issues, notice, where the state uses "rights" language, not to raise the moral bar, but to lower it forcibly - liberal Christianity has already capitulated to the state's modern ambition. It is without force, without hope, and without energy.
In contrast, the prophetic Christianity of the New Evangelization, both Catholic and non-Catholic, alone has the intellectual and moral capacity to curtail the modern state. In opposing fornication, adultery, abortion, and homosexuality, it correctly identifies the wedge issues through which the self-regulating state is inexorably attempting to expand its use of authority. To the extent that the Church achieves some success in resisting these social changes, it forces the state not to change the meaning of "rights" language, and to be accountable for its use of power. It therefore re-establishes the proper relationship of moral power between it and the state.
Finally, no matter how much things have changed since Pope Gelasius I at the end of the 5th century, three concrete principles still apply and must be observed by all parties:
1. The state has no power over the Church's sacramental life. This inviolable principle is dangerously at play in current marriage politics. "Tyranny" is not too strong a word for the state legistlating sacramental life. We, as clerics, have to be willing to stand on this principle and take the scorn of the sick world, like the martyrs before us - most especially on those critical wedge issues where the state tries to make inroads where it really doesn't belong.
2. The Church isn't better at politics that the politicians. Obvious, one would think, but useful to remind ourselves that our mission is souls, not politics. To be truly prophetic, one must be apolitical. Pope Benedict said some interesting things last week in Germany about the Church embracing its modern poverty (no Papal States, no great reserves of wealth like the medieval Church had, dependent on the charity of the people for its operating resources) precisely for this prophetic end.
3. Every good law is supported by the universal moral law, what Catholics call "natural law." This is the right relation between Church and state. The Church has both the right and the obligation to explain both the natural law and the divine law, revealed in Scripture; and to expect that the State will listen to this wisdom. The State simply cannot be its own moral voice; it needs the Church to teach it the natural law, and to point to the revealed law, over and over again.