It seems you are asking about the background and reasons of anti-Jewish and/or antisemitic sentiment throughout history. Posts of this type are common on the subreddit, so we have this reply which is intended as a general response that provides an overview of the history of antisemitic thought and action.

The essential point that needs to be emphasized: the reason for anti-Jewish hatred and persecution has absolutely nothing to do with things Jewish men and women did, said or thought. Religious and racial persecution is not the fault of the victim but of the persecutor and antisemitism, like all prejudices, is inherently irrational. Framing history in a manner that places the reason for racial hatred with its victims is a technique frequently employed by racists to justify their hateful ideology.

The reasons why Jews specifically were persecuted, expelled, and discriminated against throughout mainly European history can vary greatly depending on time and place, but there are overarching historical factors that can help us understand the historical persecution of Jews - mainly that they often were the only minority available to scapegoat.

Christian majority societies as early as the Roman empire had an often strained and complicated relationship with the Jewish population that lived within their borders. Christian leaders instituted a policy that simultaneously included grudging permissions for Jews to live in certain areas and practice their faith under certain circumstances but at the same time subjected them to discriminatory measures such as restrictions where they could live and what professions they could practice. The Christian Churches – Catholic, Orthodox, and later Protestant – also begrudgingly viewed the Jews as the people of the Old Testament but used their dominant roles in society to make the Jewish population the target of intense proselytization and other them further by preaching their fault for the death of Jesus.

This dynamic meant that Jews were the most easily recognizable and visible minority to point fingers at during a crisis. This can be best observed with the frequent accusations of "blood libel" – an anti-Semitic canard alleging that Jews murdered Christian children to use their blood in religious rituals – in situations where Christian children or adults disappeared, the communal panic immediately channeling itself as Jew-hatred with tragic results. Similarly, religious, ideological, and economic reasons were often interwoven in the expulsion of Jews to whom medieval rulers and kings owed a lot of money; in fact, one intersection of crisis-blaming and financial motive occurred during the Black Death, when local rulers were able to cynically blame Jews for the plague as an excuse for murdering and expelling them.

These processes also often took place within negotiations between social and political elites over state formation. One of the best examples is the expulsion of the Jewish population from Spain by the rulers of Castile and Aragon after the Reconquista in 1491. Expulsion and forcible conversions progressed toward an institutionalized suspicion towards so-called New Christians – Jews who’d recently converted– based on their "blood". This was an unprecedented element in antisemitic attitudes that some scholars place within the context of Spanish rulers and nobility becoming engaged in a rather brutal state formation process. In order to define themselves, they chose to define and get rid of a group they painted as alien, foreign and different in a negative way – as the "other". Once again Jews were the easily available minority.

Jews long remained in this position of only available religious minority, and over time they were often made very visible as such: discriminatory measures introduced very early on included being forced to wear certain hats and clothing, be part of humiliating rituals, pay onerous taxes, live in restricted areas of towns – ghettos – and be separated from the majority population. All this further increased the sense of “other-ness” that majority societies experienced toward the Jews. They were made into the other by such measures.

This continued with the advent of modernity, especially in the context of nationalism. The 19th century is marked by a huge shift in ways to explain the world, especially in regards to factors such as nationalism, race, and science. To break it down to the essentials: the French Revolution and its aftermath delegitimized previously established explanations for why the world was the way it was – a new paradigm of “rationalism” took hold. People would now seek to explain differences in social organizations and ways of living between the various peoples of the world with this new paradigm.

Out of this endeavor to explain why people were different soon emerged what we today understand as modern racism, meaning not just theories on why people are different but constructing a dichotomy of worth out of these differences.
A shift took place from a religious othering to one based more on nationality - and thereby, in the minds of many, on race. In the tradition of völkisch thought, as formulated by thinkers such as Gobineau and Houston Stewart Chamberlain, races as the main historical actors were seen as acting through the nation. Nations were their tool or outlet to take part in Social Darwinist competition between the races. The Jews were seen as a race without a nation - as their own race, which dates back to them being imperial subjects and older stereotypes of them as "the other" - and therefore acting internationally rather than nationally. Seen through this nationalistic lens, an individual Jew living in Germany, for example, was not seen as German but was seen as having no nation. For such Jews, this meant that the Jewish emancipation that Enlightenment brought provided unprecedented freedom and removed many of the barriers that they had previously experienced, the advent of scientific racism and volkisch thought meant that new barriers and prejudices simply replaced them.

Racist thinkers of the 19th century augmented these new barriers and prejudices with conspiratorial thinking. The best example for this antisemitic delusion are the Protocols of the Elders of Zion, a fake political treatise produced by the Tsarist Secret Police at some point in 1904/05 which pretends to be the minutes of a meeting of the leaders of a Jewish world conspiracy discussing plans to get rid of all the world's nations and take over the world. While the Protocols were quickly debunked as a forgery, they had a huge impact on many antisemitic and völkisch thinkers in Europe, including some whose writings were most likely read by the young Hitler.

The whole trope of the Jewish conspiracy as formulated by völkisch thought took on a whole new importance in the late 1910s, with the end of WWI, the Bolshevik revolution, and subsequent attempts at communist revolution in Germany and elsewhere. Jews during the 19th century had often embraced ideologies such as (classical) liberalism and communism, because they hoped these ideologies would propagate a world in which it didn’t matter whether you were a Jew or not. However, the idea of Jews being a driving force behind communism was clearly designed by Tsarist secret police and various racists in the Russian Empire as a way to discredit communism as an ideology. This trope of Jews being the main instigators behind communism and Bolshevism subsequently spread from the remnants of Tsarist Russia over the central powers all the way to Western Europe.

This delusion of an internationalist conspiracy would finally result in the Nazis’ Holocaust killing vast numbers of Jews and those made Jews by the Nazi’s racial laws. While this form of antisemitism lost some of its mass appeal in the years after 1945, forms of it still live on, mostly in the charge of conspiracy so central to the modern form of antisemitism: from instances such as the Moscow doctors’ trial, to prevalent discourses about Jews belonging to no nation, to discourses related to the Israeli-Palestinian conflict, to the recent surges of antisemitic violence in various states – antisemitism didn’t disappear after the end of the Holocaust. Even the Protocols of the Elders of Zion, the conspiratorial pamphlet debunked soon after it was written at the beginning of the 20th century, has been consistently in print throughout the world ever since.

Again, anti-Jewish persecution has never been caused by something the Jews did, said, or thought. It was and is caused by the hatred, delusions, and irrational prejudices harbored by those who carried out said persecution. After centuries of standing out due to religious and alleged racial difference, without defenders and prevented from defending themselves, Jews stood out as almost an ideal “other.” Whether the immediate cause at various points has been religious difference, conspiracy theory, ancestral memory of hatred, or simply obvious difference, Jews were and continue to be targeted by those who adhere to ideologies of hatred.

Further reading:

Amos Elon: The Pity of It All: A History of the Jews in Germany, 1743-1933. New York 2002.

Peter Pulzer: The rise of political anti-Semitism in Germany and Austria, Cambridge 1988.

Hadassa Ben-Itto: The Lie That Wouldn't Die: The Protocols of the Elders of Zion. London 2005.

Robert S. Wistrich: Antisemitism: The Longest Hatred. New York 1991.

"Since they looked like other people and were not vocal about their religious background"

That was the result of Jewish Assimilation In Germany, which came about because of Jewish Emancipation, which started in 1812 and was only fully completed in 1919, with the fall of the German Empire at the end of WWI.

Historically, throughout all of Europe, Jews were always persecuted and oppressed. This was also true in the Holy Roman Empire of the German Nation, which lasted from 800 to 1806 and had multiple discriminatory restrictions on Jews. This changed when Napoleon defeated the HRE. One of the first things he did was Emancipate Jews and lift restrictions on them, allowing them to participate in any career or field of their choice, among other things. Even after Napoleon was defeated, the various German Kingdoms/Provinces would continue the Process of Jewish Emancipation, with varying degrees of success.

There are two important things to keep in mind.

One, Emancipation led to German Jews assimilating. For example, after Emancipation, they were required to take last names: one of the most common name choices were Germanic sounding names, like "Berg". Also, because of Emancipation, many Jews from more restrictive places - especially Poland and the Russian Empire - fled to Germany in search of a better life. This is one of the reasons: classic Xenophobia, i.e. "strange" or "foreign" people moving in in large numbers. In this case, it was Eastern European Jews moving westwards in search of a better life.

Which leads us to the second; the Fall of the Holy Roman Empire also led to the fall of Classic Feudalism and the start of the Industrial Revolution. As part of this, religion became less important and education became more and more secular. German Christians struggled to adapt to this, but German and Eastern European Jews had an advantage due to their culture investing heavily in education. Indeed, because of restrictions on their rights - up to and including literal mass murder in the form of Pogroms - education, whether religious or secular, was all Jews had. This allowed them to adapt better to industrialization, and thus climb the Social ladder. This also exacerbated point one, because as mentioned before, Emancipation of Jews encouraged more Jews to move in, which led to increased Antisemitism.

In other words, the reason for Pre War German Antisemitism can be compared to violence against Black People in the Post Civil War era. After the end of Slavery, American Black People tried and in many cases, succeeded to better themselves and climb up the Social ladder. Hence, we had things like the Black Wall Street in Tulsa, one of the most prosperous Black Neighborhoods/Communities in the Country. White Americans felt threatened by Black Prosperity and wanted a return to the days of Slavery, when Black People "knew their place", hence the Tulsa race riot and American laws designed to cheat Black People.

German (And European) Antisemitism in General was similar. Indeed, many of the German Kingdoms instituted laws that limited how Many Jews could live in such and such place and continued to place restrictions on the Careers they could pursue or properties they could own. Until the end of WW1, Jews were restricted from the Upper Ranks of the German Military and the Civil Service. This was because Germans (and other Europeans) saw Newly emancipated Jews as an economic threat, not helped by their being "foreign"/"Alien". This perception was especially fed by extremely poor Jewish Refugees from Eastern Europe, many of whom kept going to Germany/Austria until the eve of WW1. Hitler himself would talk about the extremely poor Jewish Refugees in Linz.

Antisemitism isn't really that different from other forms of Racism. Antisemites simply don't like Jews, and their "dislike" was highly influenced by the perceived "threat" Jews posed to them economically. Which was why many of the early Nazi Era Anti Jewish Laws were economic in nature, designed to reverse what the Nazis believed were "Ill gotten gains"

Source Gotz Aly, why the Germans, why the Jews? Envy, Race Hatred and the Prehistory of the Holocaust.

In addition to the excellent overview posted by u/mimicofmodes consider I have a question about German anti-semitism written by u/kieslowskifan which talks about the difference between German and Nazi antisemitism and how the latter emerged and took hold