The correct answer is that non-Messianics fear they missed the Messiah that Messianics follow..
It has been about 2500 years since non-Messianic Jews heard from God through a prophet....That should be enough to unnerve anyone about their position when the Tanach is a record of where God speaks and acts to correct or expand His people's understanding of Him. The Torah and Tanach exist because of the thousands of examples of God speaking and acting, it suddenly stops....and has not restarted according to non-Messianics. This is truly a puzzle and a concern.
Messianic Jews state that God still does speak and act on behalf of His people. So if you aren't filled with rage and hatred, a rational, educated person would ask how they could have what the Messianic Jews have.
But the name Messianic Jew reveals how to have what they have...believe in the Messiah. To admit the Messiah has come is to admit that the non-Messianic Jews have been wrong about Yeshua for 2000 years.
Stan Telchin wrote this when his daughter decided to follow Jesus:
“Well, I’ve become a believer, too.”
There was a long moment of silence.
“What does that mean, Judy?”
“It means that I believe in God. I believe that the Bible is the Word of God, and (long pause) I believe that Jesus is the Messiah!”
I was speechless.
Many parents might have welcomed Judy’s words, but they absolutely crushed me!
You see, we are Jewish!
To mention the name of Jesus is awkward enough. To consider Him as the Messiah is something we just do not do. For any of us to believe that Jesus is the Messiah is to betray our people, to join the enemy and to desecrate the memory of all our ancestors over the last two thousand years.
How could Judy do this to us?
Rage started to well up within me.
“Betrayed” copyright 1981, 2007 (pg 14)
But before we can say that non-Messianic Jews deny Messianic Jews their Jewishness, what makes a Jew a Jew needs to be defined?
1. Are Jews born into Judaism?
2. Do they have to make a public profession in front of other members accepting their scriptures as their own?
3. Or is it simply being a member of a community?
PaperbackWriter JPA KosherNinja defines Judaism:
***Judaism is like every other faith: we have members who are very strict in adhering to the letter of the law, and we have members who are less so. A person who is born to a Jewish mother is Jewish, whether they never set foot in a synagogue or whether they become a Rabbi - they are EQUALLY Jewish.***
According to her, Judaism is determined by birth. Because she continues and says they can ***never set foot in a synagogue*** and be a Jew.
Therefore, there is a sect of Judaism that believes that you are born into Judaism.
If one claims the scriptures as the way of determining Judaism, then Ahab would be a get place to start. He brought Baal worship into Israel and married a Gentile who delivered a son. If the Tanach has any value, then the fact that Ahab's son is considered a Jew should be all that is needed to prove Judaism is determined by birth.
Interesting enough, Elijah tells God how Ahab all Israel turn to worshipping Baal a few days after killing the prophets of Baal. (1 Kings 19)
Here is the classic scriptural example of what should happen when a Jew believes differently that Judaism....nothing. All of Israel was NOT called "apostate" or "Baalians." There is not a word about classes reaffirming a faith that God. In this scriptural example, there isn't one shread of evidence to support the practice of non-Messianics.
So the scriptures support your position, "A Jew is a Jew by birth." While welcomed, conversions of Gentiles to non-Messianic Judaism is rare. However, non-Messianic Jews deny this right of conversion to Messianic Jews.
Therefore, they won't give a scriptural text to support their position.... though some might reference
Ex 32:***'Whoso is on HaShem'S side, let him come unto me.' And all the sons of Levi gathered themselves together unto him. 27 And he said unto them: 'Thus saith HaShem, the G-d of Israel: Put ye every man his sword upon his thigh, and go to and fro from gate to gate throughout the camp, and slay every man his brother, and every man his companion, and every man his neighbour.' 28 And the sons of Levi did according to the word of Moses; and there fell of the people that day about three thousand men.*** http://www.hareidi.org/bible/Exodus32.ht...
One can hardly confuse what Moses commanded be done to those who worshipped the golden calf with what occurred with Ahab or is done with Messianic Jews.
Stan answers your question when he says, "For any of us to believe that Jesus is the Messiah is to betray our people, to join the enemy and to desecrate the memory of all our ancestors over the last two thousand years."
So the intense hatred of Messianic Jews is identified as it is seen as betrayal. This concept of betrayal isn't taught by the Hebrew scriptures. There is no reference in the Tanach of this betrayal.
Therefore, this hatred is taught from an extra-Biblical source which is considered equal or superior to the Tanach....which is why non-Messianics don't like quoting their scriptures to support their position.
As Michelle R says,
***The problem with applying one religion's yardstick to another religion's belief system, is that it will never measure up. Orthodox Jews may believe that Reform Jews are not practicing true Judaism because they don't see the laws as binding***
Messianic Jews aren't the only ones accused of not practicing "true Judaism" by those who are divisive.
Hope this helps
EDIT
Marcuss: ***Well, the problem with doing that is you will be going to someone who is lying about what they are and thus you will get nothing but lies from them.***
The problem non-Messianic Jewish answers are they make big claims like calling MJs "lying" and then expects everyone to take what you say by blind faith. It should be simple to quote someone who has heard the voice of the God you claim is real but who non-Messianics have never actually heard or seen....so on what basis can the non-Messianics claim they are currently following God given they had prophets, scriptures and Solomon's Temple prior to conquest of Babylon. The Jewish scriptures bear witness through the prophets that the reason for the conquest of Israel was due to disobedience.
So how can the destruction of Herod's Temple be a result of obedience when the High Priest was selected by Rome and not the way the Torah mandated? It can't....which is why the non-Messianic position can't be defended so their highest defense is name calling.