I thought this said Martin Luther King at first, and balked.

Martin Luther was indeed a passionate anti-Semite, but that was only in the later part of his life -- at least, the vituperation only showed itself at this point. Early on, his view on Jews seemed to be one of compassion and an earnest desire to convert Jews to Christianity. He wrote to his friend, Georg Burkhardt, in 1519:

"Absurd theologians defend hatred for the Jews. … What Jew would consent to enter our ranks when he sees the cruelty and enmity we wreak on them—that in our behavior towards them we less resemble Christians than beasts?"

He was particularly against the prevailing Servitus Iudaerom, a doctrine in Europe taken from the Corpus Iuris Civilis of Justinian the Great, regarding Jews and their state of servitude and second-class citizenship.

A bit later, in 1523, he wrote an early essay called "Jesus Christ was Born a Jew," and urged his fellow Christians thusly:

If I had been a Jew and had seen such dolts and blockheads govern and teach the Christian faith, I would sooner have become a hog than a Christian. They have dealt with the Jews as if they were dogs rather than human beings; they have done little else than deride them and seize their property. When they baptize them they show them nothing of Christian doctrine or life, but only subject them to popishness and mockery...If the apostles, who also were Jews, had dealt with us Gentiles as we Gentiles deal with the Jews, there would never have been a Christian among the Gentiles ... When we are inclined to boast of our position [as Christians] we should remember that we are but Gentiles, while the Jews are of the lineage of Christ. We are aliens and in-laws; they are blood relatives, cousins, and brothers of our Lord. Therefore, if one is to boast of flesh and blood the Jews are actually nearer to Christ than we are...If we really want to help them, we must be guided in our dealings with them not by papal law but by the law of Christian love. We must receive them cordially, and permit them to trade and work with us, that they may have occasion and opportunity to associate with us, hear our Christian teaching, and witness our Christian life. If some of them should prove stiff-necked, what of it? After all, we ourselves are not all good Christians either.

However, over the next 15 years or so, Luther's attitude toward the Jews changed. We don't exactly know why this is, but we know, for example, that he lived in a community where Jews had been expelled for about a century, and apparently felt this was a good thing -- either because of his experience in the community, or what he was told about it prior to the Jews having been expelled. We can also surmise from some of his writings that he either personally (unlikely) or through hearsay, experienced refusals of Jews to convert to Christianity or assist Christians or something along these lines, and this frustrated him.

1536 is considered by many historians to be the turning point in Luther's feelings about Jews. At this time, Luther was a prominent member of the court of John Frederick I, Elector of Saxony (styled John the Magnanimous), who was also known as the "Champion of the Reformation," and who provided Luther with a great deal of funding and protection. John was very anti-Semitic, and frequently issued polemics against Jews.

In 1536, John issued an edict prohibiting Jews from living, doing business, or even passing through his realm. This caused a great deal of strife for the Jews around this place at this time, and the leader of the Jews in the Holy Roman Empire, whose name I cannot remember, petitioned Luther to ask the Elector to loosen his edict. In response, Luther replied:

"... I would willingly do my best for your people but I will not contribute to your [Jewish] obstinacy by my own kind actions. You must find another intermediary with my good lord."

Clearly Luther was frustrated by the Jews' unwillingness to assimilate. However, he may also have been influenced by the prevailing thoughts of those around him at the time. It's unlikely that Luther had much direct contact with Jews, but John Frederick I, and much more importantly, Holy Roman Emperor Charles V, had received a great deal of financial support from Jewish sources. The Emperor treated the Jews with some kindness as a result, but the opposite was true of the Elector of Saxony. And indeed, many Lutherans may have felt -- at complete odds with history -- that the Catholic establishment was in league with or on the side of the Jews, and vice versa, and reacted against them accordingly.

Luther's treatment of the Jews worsened from there. He issued various letters and polemics railing against the Jews, and indeed, the city of Strasbourg eventually banned the dissemination of Luther's books, after Lutheran pastors there began to exhort their parishioners to murder the Jews en masse.

It's not clear exactly what experiences Luther had with Jews himself, but he may have had a lot of second-hand experience through contacts and friends, much of it leaving him with a negative feeling toward Jews. Indeed, the level of vituperation suggests little direct contact, as it is hard, generally speaking, to hate personally those with whom you have regular intercourse.

Luther's views were probably also affected by his readings and the general tenor of talk at the time. A Jewish convert to Christianity named Anton Margaritha wrote a book in 1530 called "The Whole Beliefs of the Jews," which was virulently anti-Semitic. Luther is known to have read this work, despite the fact that it was publicly discredited and banned in the HRE.

So, that's a long way of saying: we don't know for sure what kinds of experiences Luther himself had with Jews, but we do know that something -- probably something gradual -- changed his views from hopeful pity to vicious hatred around the early-mid 1530s.