Late conservative rabbi Jacob Neusner said “Jews who practice Christianity cease to be part of the ethnic Jewish community, while those who practice Buddhism remain within.”
Look, most modern Christians see the Jews as the ancient religion of Israel. The spiritual descent of the ancient Israelites that didn’t accept Jesus as Messiah. As if Jews had been stuck in the Old Testament. If this were true, it would be possible to be Jew and Christian at the same time. For example, James the Just, practiced the religion of the Old Testament thoroughly while being one of the first Christian leaders.
In reality, modern Judaism is a new religion founded in the first centuries of the Christian era. Its base is not the Old Testament but the Talmud, a book of commentaries that replace the Mosaic law in the Old Testament (which was unable to be applied after the destruction of the Temple in 70 AD, since the Mosaic law required a Temple). You can know more about Judaism in this article, written by a Jew.
For example, traditionally religious Jews pay little attention to most of the Old Testament, and even very learned rabbis or students who have devoted many years to intensive study may remain largely ignorant of its contents. Instead, the center of their religious world view is the Talmud, an enormously large, complex, and somewhat contradictory mass of secondary writings and commentary built up over many centuries, which is why their religious doctrine is sometimes called “Talmudic Judaism.” Among large portions of the faithful, the Talmud is supplemented by the Kabala, another large collection of accumulated writings, mostly focused on mysticism and all sorts of magic.
This new Talmudic religion is anti-Christian. I don’t know how to put this in a non-harsh way and I don’t mean to offend anybody. Talmudic Judaism was born as a reaction against Christianity, to which most Jews had converted (especially in the diaspora, see the work of Rodney Stark about the growth of early Christianity). It was necessary for it to protect against the assimilation of Christianity, which had become dominant in the late Roman Empire.
Whereas pious Muslims consider Jesus as the holy prophet of God and Muhammed’s immediate predecessor, according to the Jewish Talmud, Jesus is perhaps the vilest being who ever lived, condemned to spend eternity in the bottommost pit of Hell, immersed in a boiling vat of excrement.
Religious Jews regard the Muslim Quran as just another book, though a totally mistaken one, but the Christian Bible represents purest evil, and if circumstances permit, burning Bibles is a very praiseworthy act.
Pious Jews are also enjoined to always spit three times at any cross or church they encounter, and direct a curse at all Christian cemeteries. Indeed, many deeply religious Jews utter a prayer each and every day for the immediate extermination of all Christians.
Look, I am not attacking the Jews (most Jews don’t know their religion that well). I only say that their religion excludes Christianity. This is why Messianic Jews are (Protestant) Christians and not Jews.