Christians often ask why Jews get so irate when they "share" their faith with us. (To Christians: there are millions of lapsed Christians, billions of Muslims, millions of Hindus, many, many non-Christians, so why concentrate on a few million Jews who just want to be left alone?) To answer this, let me share some Jewish history with you.
G-d made an eternal covenant with our father Avraham, saying: 'Get thee out of thy country, and from thy kindred, and from thy father's house, unto the land that I will show thee. And I will make of thee a great nation, and I will bless thee, and make thy name great; and be thou a blessing. And I will bless them that bless thee, and him that curseth thee will I curse; and in thee shall all the families of the earth be blessed.' (Gen. 12:1-3). Throughout the ages, G-d has been with us through fire and water, Crusades and pogroms, jihad and terror. Yet, many Jews did not merit to have Jewish descendants survive until today.
When the Jews left Egypt, four-fifths of the Jews did not leave but perished in the plague of darkness. Only 20% of the Jewish people were ready to defy Egyptian idolatry by sacrificing a goat and to leave the exile. If you are a Jew today, you are descended from those fifth of amazing Jews who had the courage to follow HaShem into the desert.
When the Jews came into the land of Israel, the ten northern tribes abandonned the Torah and worshipped idols. While the tribes of Judah and Benjamin stayed mostly loyal to the ways of their ancestors, the kings of Israel built golden calfs and offered their children to Molech. These ten tribes were exiled by Sancheriv and, until the coming of the moshiach, have disappeard. If you are a Jew today, you are descended from those two tribes that kept G-d's laws.
When Israel was conquered by the Greeks, many Jews were seduced by Greek Hellenic culture. These Jews hellenized their names, accepted the Greek idols and emersed themselves in Greek culture. Many even undid their circumcisions with the hopes of fitting in. When the Jewish majority that kept the "old-fashioned" ways drove the Hellenizers out in the miracle of Channukah, the Hellenizers were free to move to Greece were they could live out their days, for all intents and purposes, as good Greeks. If you are a Jew today, you are descended from the brave Chassidim and the brave Jews who fought alongside Judah the Maccabee and the Hasmoneans.
When the Hasmoneans reestablished Jewish sovereignty over Israel, many of the Jewish elite and priestly class joined the Sadducees religious sect. The Sadducees believed only in the literal Torah text even as the majority of the people stayed true to the teachings of the Sages, to Pharaic Judaism which came from Moses at Sinai. After the destruction of the Temple, these heretics disappeared from history. If you are a Jew today, you are descended to those who refused to deviate from the laws and teachings of Moshe Rabeinu A"H.
When the Romans conquered Israel, in an atmoshpere of intense Messianic anticipation, some Jews turned to heresy and followed, alongside the Sadducees, the Essenes and early Notzrim who later became known as Christians. The Essenes rejected mainstream Jewish society, which they viewed as corrupt, and went to live in commnunal settlements near the Dead Sea. Other Jews joined the Christian movement and believed in a divine intermediary, in the form of the Christ, and called the rabbis 'broods of vipers'. These groups either vanished or assimilated into the gentile Church. If you are a Jew today, you are descended from those who were not swayed by the Essenes, Notzrim or any other heretic.
When the Jews came to Babylon and the Amoraism started to set down the Talmud, a large segment of the Jewish population came to follow the teachings of Anan ben David and became Karaites. They believed only in the literal written word of the Torah and saw the rabbis as false, adding to Moses's law. Besides a few thousand families of Karaites in Israel, they are largely gone. If you are a Jew today, you are descended from the Jews who accepted both the Written and Oral Torah as divine and originating at Sinai.
When the Jews came to Europe, they were attacked by Christian Crusaders who demanded that they accept baptism or death. Entire communities recited the Sh'mah, the traditional Jewish prayer which affirms G-d's Oneness, before taking their own lives or being butchered by Crusading swords. Only a handful of Jews converted. If you are an Ashkenazi Jew today, you are descended from those who refused to worship a foreign god.
When the Jews were given the choice between expulsion from Spain or conversion, tens of thousands of Jews choce Spain and Christianity over the Torah. Many thousands of those continued to practice Judaism in secret, called derogatorily marranos which means 'pigs' (though properly called anusim, 'those who were raped') until being burnt by the Inquisition. Of the tens of thousands who compromised the New Christians, none remain as part of the Jewish people. In fact, many descendants of anusim today are returning to the faith of their ancestors. If you are a Sephardic Jew today, you are descended from those Jews who prefered leaving the richness of Spain than compromising their Jewish faith.
In every age, only those who kept the Torah of Moses, in both its written and oral forms, have survived. Even today, it was found in a study of Jewish demographics, that out of 100 Reform Jews, within 3 generations, only 4 remain Jewish, while out of 100 Orthodox Jews, within 3 generations, there are close to 3000 Jews. It has been estimated that within 50 years, the Reform Jewish population of the United States will have decreased by 90% through assimilation and intermarriage. That's why anyone who wishes to separate a Jew from Moses' Torah is our enemy. Christian missionaries, whether through the sword and stake or through proselytizing and "Christ in the Passover" seders, have the same ultimate goal, or at least result: the complete eradication of the Jewish People. Jews, your ancestors gave so much to remain Jewish, and those who did not are no longer. Do not sell your birthright for a pot of lentils.
Shoovah Yisrael as HaShem Elokeicha! "Return O Israel unto HaShem your G-d!" (Hoshea 14:1)