Additional question, which in some way is the flip side of this: I've heard it said that Judaisim was the only non-Christian religion allowed in Christendom. Is largely true, and if so, how did it come about that Judaism had that degree of acceptability?

Incredibly broad question. Antisemitism has existed in so many places and in so many permutations that big long books have been written about the subject and only scrape the tip of the iceberg.

Additionally, this is the sort of historical question where the bias of the historian can have a huge impact on the answers you receive. For a good scholarly analysis of the development of antisemitism in early Christian Europe, check out James Carroll's book "Constantine's Sword," which was published in 2007 and should be available at most local libraries.

This is by no means intended to be a complete answer, but there are two common themes to most theories and histories as to the origins of anti-antisemitism, and they are primarily about from where they are derived. The first is what we might call the money approach. This is driven by the (somewhat flawed) belief that Jews made up the money-lending business in Europe, and thus controlled the debts of people. I am not sure about you, but I don't know too many people who like bankers these days. I'll come back to that point later. Although it's not strictly speaking true that the only moneylenders were Jews, they certainly made a brisk business in that trade, and in general were regarded as well moneyed. Part of the reason for this was, of course, that there were restrictions on their purchase or property (even more so than a non-noble gentile would face) and thus no real good way to use their gains from the capital they acquired, other than lending it out again or spending it on luxuries. One might suspect how these factors might build resentment in any case.

The second stems far far back, way earlier than the medieval role as money-lenders, and that is their 'separate' status. During the Roman Period, Jews were that pesky people who refused to recognize the diefied status of the Emperor, and in general just didn't seem to know their place. They also had a habit of spreading throughout the Empire and congregating into their own small little communities within cities. They had different believes, refused to assimilate to the local culture, and in general were just different. Let's fast-forward to medieval Europe. Here we have similar things going on, but in this case, much of it was imposed. Almost every city in Europe with a Jewish population required that they be separated from the local population, actively encouraging a lack of assimilation. Above all, they didn't convert, or when they did, it was constantly questioned. Jews used this to their advantages in many cases, and 'Jewishness' represented a trans-political identity that gave them many advantages in the business sphere because wherever a Jew went, it's likely he could find a Jewish family in the city to do business with and make connections. We can see how this could also cause problems down the road as this idea crystallized in the heads of people when nationalism rolled along.

The reality of antisemitism in practice though is that it's most dire consequences on local Jewish populations can almost always be linked to a crisis elsewhere where these people become the scapegoat, or the fall guy. If a plague breaks out in your city, who are you going to blame first? The outsiders who have known contacts with other cities with the plaugue. If your people (and particularly the crown) are drowning in debt, who are you going to blame first? The people who hold that debt. And unlike the nobility of your country (who you might also owe debt to) nobody is going to complain when you banish (or kill) them.

There is also one ancillary note that I feel is more an 'excuse' than necessarily a cause of antisemitism, and that is the 'they killed Jesus!' one. Sometimes this gets repeated by modern antisemites, and seems particularly common among WASP Neo-Nazis, but it seems to be an excuse rather than a cause.

One other thing to touch on in relation to the 'they didn't convert' comment. It was not merely Jews who faced this type of prosecution. One case study that I highly recommend is in Medieval Castile during the Reconquista. Both Muslims and Jews alike were given opportunities to convert, and many did. But many of those that did faced constant scrutiny and harassment. The reason being: if you were found to be a false convert, you not only lost your privileges, you were often banished and allowed only to take what you could carry on your back. I highly recommend reading a history of the Sephardic Jews to get a pretty decent idea of how Jews were treated in harsh circumstances, and the rationale as to why. In general, we can see that the reasons for the lower castes to hate them were likely rooted in resentment and fear of differences, while the upper caste's hate was likely more rooted in naked self-interest. Fomenting the hatred in the former by the later explains a lot of the incidences of direct violence we know of in the Medieval period.