The church tax was paid by the church members. No one was forced to be Christian. Exceptions existed, but the model was that of voluntary participation.
Secularism is what brought us oppressive government. Why? Because religion is essentially voluntary: its laws are binding on its adherents and are merely advice to others. The government, in contrast, has a monopoly of power, which is claimed territorially. You could be a Jew or a Muslim in Medieval France, but you cannot pledge allegiance to Spain in the French Republic today, -- you will have to move to Spain. Once the legal realm grew separately from the religious realm, the law became imposed territorially by force and either we got government-enforced ethics or government-enforced evil. Neither is a very good deal for the individual.
Differently put, feudalism was all about voluntary allegiances, to the suzerain and to the church. This is why the libertarians should study the Middle Ages as their model, rather than assume that the French or American republics were their models. There was much good about the American republic (nothing good about the French one), but its rootedness in the false ideals of the Enlightenment doomed it to failure: it only survived a few decades.
Your idea that it was secularism that brought us oppressive government does not measure up with history. While it is true that since the secularists pretty much won out politically over the the Christians by the early mid 19th Century, the matter did not end. Since then both pseudo liberal and pseudo conservative Christians alike, have been continually attempting (often quite successfully), to make their various Christian doctrines a part of our secular laws. They have often done so by joining forces with specific secular forces on an issue by issue basis. Thus, blame does not lie particularly with either the Christians or Secularists.
Your idea that one could be a Jew or a Muslim in Medieval France, is only periodically and circumstantially true under very restrictive conditions, and thereby can also be said to be quite wrong. Your idea that feudal allegiances were voluntary, takes the word voluntary to such a new level of understanding, as to render it a whole new meaning.
...libertarians should study the Middle Ages as their model, rather than assume that the French or American republics were their models.
I do not agree that either should be studied as "their model." But both do need to be studied as "a model." As I have viewed it, one or a combination of several varying models of the former, as well as various forms of corporate models (both feudal and non feudal), are far more likely than any of the later two models. But those are not the only models.
It is however, my opinion that the current popular libertarian visions (if accepted, adopted or applied), would result in something much different than intended. I say shame on the Libertarian Party for brushing such considerations and studies under the rug in its rush to gain quick easy conservative votes. The route of least resistance, is not the route of greatest advantage.