I can't, because it doesn't make any sense.
I'm going to guess, based on how you've acted in this thread, that you're on the younger end.
And there's nothing wrong with that, but a big thing people need to come to terms with as they leave childhood and enter adulthood is that the world does not, and will not, conform to how you think it "makes sense."
No, Christian persecution of Jews doesn't "make sense" if you look at it from the perspective of someone in 2023 looking back, but A LOT about the world doesn't make sense from a modern perspective. So let's talk about the roots of antisemitism, specifically among Christians.
Yoshua ben Yosef aka Jesus of Nazareth was Jewish and primarily preached to Jews. He wasn't indifferent to Romans or other non-Jews, but his original message is very much tailored to Jews of first century Judea, dealing with theological and socio-political concepts they'd have been aware of.
His original followers were, likewise, Jewish. And this wasn't lost on the Roman occupiers of Judea. Pilate, the Roman governor, originally sent Jesus to King Herod since as a Jew he was Herod's problem. And following Jesus' crucifixion Romans considered Jesus' followers just another sect of Judaism. And that made sense. His followers were all still Jews, and Jesus' theological basis for being the Messiah was rooted in Jewish prophecy. It seemed like just one more sect of Judaism at a time when Judaism was divided into a lot of sects as-is.
Early Christians thought this way too, believing that Jesus fulfilled the prophecy of the Messiah mentioned in the Torah and attempted to convert other Jews to their thinking. This was- again- a purely internal Jewish sort of thing. With one sect of Jews- Christians- trying to convince the rest of the Jews that they were right.
It became clear, however, that Jesus' followers were not going to be able to convert the rest of the Jews. As said above, Judaism was very fractured among many sects at this time and while Jesus had his followers they were but one of many. It's around the time Saul-turned-St. Paul that things began to shift. Paul likely wasn't the big L-Leader of this movement, but he was a primary advocate of Christianity breaking away from Judaism. At its root it was a frustration among early Christians that their fellow Jews wouldn't accept Jesus as the Messiah, but there were other layers.
As Christians realized their fellow Jews weren't likely to convert they began to target Greeks, and made significant headway into the Greek population of the near East. This was made easy by the fact that the Christian scriptures that would become the New Testament were written in Greek, as Greek was the lingua franca both Jews and Romans could understand.
And something interesting happens here in the historical record. Christian community leaders start telling their followers to stop attending synagogue services, and Christian community leaders start declaring that a "gentile" who converts to Christianity doesn't have to get circumcised. Circumcision is a Jewish rite, and early Christians originally believed that any non-Jewish converts to Christianity would first have to get circumcised as Jews. But starting around the time of Paul this changes. The religion is making in-roads into Greece and suddenly Christian leaders are talking about no longer attending Jewish religious services and removing a very Jewish religious rite to make recruiting non-Jews easier.
All of this was done because Christians realized that 1) most other Jews were unreceptive to the idea that Jesus was the Messiah 2) therefore the faith should be spread outside of the Jewish community and 3) if Romans- the authority at the time- think Christianity is a sect of Judaism then make changes to Christianity to make a clean break with Judaism.
3 proved vital, and it's through that line of thinking that we don't just see early Christians renouncing Jewish religious services and telling new converts they don't have to be circumcised, they start attacking Jews. If the ruling Empire in the area thinks you're part of a group, and you want to really convince them you're not, you attack that group to say "see we aren't them!"
It's here where Jews were blamed for Jesus' crucifixion, despite that being a very Roman method of execution, and it's here where we see early Christian leaders lashing out at Judaism on theological grounds. If you've ever heard a Christian call Judaism a "dead religion" or some variation of that, this is where that comes from. It's early Christian leaders attacking Judaism in an effort to separate themselves from Judaism.
Roughly three hundred years later Constantine the Great won a civil war in Rome and converted to Christianity because he claimed an angel told him he'd win under the sign of the Christian cross. What's interesting is that Constantine's original victory wasn't 100% decisive and you had other centres of Roman authority that attempted to use Greco-Roman paganism as a bulwark against Constantine's Christianity. And there was even a plan at one point to rebuild the Jewish Temple in Jerusalem in an effort to strengthen non-Christian faiths in the region. Constantine's total victory killed these plans, but they tell us that three centuries after Jesus' death Romans very much viewed Judaism and Christianity separately.
Anyhow Constantine's conversion ushered in a mass conversion through the Empire as everyone wanted to get on the new regime's good side. There's a lot of debate about how sudden vs how prolonged paganism's death was in this period, but it's immaterial. Fact was Rome's government was now in the hands of the Christian church and they inherited the ideological attacks Christians levelled against Jews three centuries earlier.
The fall of the Western Roman Empire in 476 AD/CE ushered in the Middle Ages and with no widespread governing authority left, the Church filled that void. The Church, inheriting earlier attacks on Jews and an inherent bias against them for not accepting Jesus, led to the Church either actively aiding in persecutions against Jews or just... looking the other way.
Theologically Jews represented something very uncomfortable for the Medieval Christian Church. On one hand they saw themselves as the inheritors of the Old Testament along with the New. The Biblical prophets, figures such as Abraham, Jacob/Israel, Moses, Aaron, right down to Judah the Maccabee were seen as the embodiment of divine virtue, men who came before and set the stage for Jesus.
And yet the Jews still existed as a distinct faith. Had the Jews all converted to Christianity this Christian "adoption" of Jewish Biblical figures would have been very easy to justify, but with Jews retaining their pre-Jesus faith and venerating these figures as central figures of their own faith and history it made Christianity's claim on that legacy just a bit tenuous.
Of course the Christians controlled all the governments and had all the armies so they could say whatever they wanted really, but theologically the continued existence of the Jews made Christian claims to have inherited the legacy of the Bible very troublesome.
Which is why you see some branches of Christianity claim that they supplanted the Jews.... it's basically spite in theological form, angrily lashing out at Jews for refusing to convert.
Enter Martin Luther and the Protestant Reformation. Luther was originally very friendly to the Jews, and declared that of course the Jews haven't converted, have you seen how corrupt the Catholic Church is?
Things got dicey after Luther's Reformation movement kicked in high gear and new sects of Christianity based around his teachings sprung up. When the Jews still showed no desire to convert that's when Luther started writing and publishing very antisemitic tracts.
And all in all...
That's what it comes down to. The first Christians were Jews, and were Christians simultaneously while being Jews. The first attempted converts were their fellow Jews and most other Jews found no compelling reason to believe that the rabbi who was just crucified by the Romans was the Messiah.
Since then Christian hostility towards Jews has been rooted in Judaism's refusal to accept Jesus as the Messiah and that Judaism's continued existence as a living religion undermines Christian claims to have inherited the Biblical legacy of Abraham, Moses, etc...
Now this isn't where the story ends. As the Enlightenment took hold in Europe religion stopped being the dominant lens through which thinkers, philosophers, politicians, and monarchs looked at the world. "Jewish emancipation," basically the repeal of Medieval laws limiting where Jews could work, how they could work, and their political rights, became a cause that picked up steam through the 18th and 19th centuries, becoming near-universal in Western and Central Europe by the dawn of the 20th century.
Yet as you can see in the present day, a relaxing of old bigotries and a spread of tolerance does not make said old bigotries go away. Antisemitism continued after this point as people raised on a religious hatred for Jews now had to find other reasons to justify said hatred. Enter pseudo-scientific racism and the eventual rise of the Nazi Party.
By then, though, the Church was undergoing a massive shift in how it viewed Jews and many individual Churches and Priests acted to save Jews from the Nazis, even if the Church itself was too deadlocked to do much if anything. By the time WWII was over and the full extent of the Holocaust known the Church finally saw fit to rid itself of antisemitic doctrine, something which was followed by other denominations. These days even the Lutheran Church outright rejects Luther's antisemitic writings.
So these days the Christianity- for the most part- has moved beyond its antisemitic issues. That doesn't mean, however, that they didn't happen. It doesn't matter if it doesn't make sense to you personally. It all happened, and there are reasons why it did.
Martin Luther wanted the Jews to believe in Jesus, who came to die on the Cross for their sins. He acted out of frustration, and went about converting Jews to Christianity the wrong way. Luther cared deeply about the Jews because he believed they were God's chosen people. He wanted them to believe in Christ and be saved. Luther was trying to save the Jews, and he acted out of love. Any other way of interpreting this would simply not make sense.
Luther acted out of love? Is that why he wrote that Jews should be killed down to the last child and their synagogues torn down to the foundations? That's love to you?
I wrote that entire (admittedly truncated) rundown on where Christian antisemitism came from under the assumption that you are someone genuinely confused and wanting answers. But this makes me think something else is going on.
So tell me. What angle are you coming at with all of these questions?
en.wikipedia.org
