Thank you for asking a question as the question reveals where you fail to understand.
1. Jesus was Jewish.
2. All His disciples were Jewish.
3. At no point in the NT does Jesus or ANY of His disciples ever renounce their being Jews.
4. At no point in the NT does Jesus or ANY of His disciples ever say they are starting a new religion and call it Christian.
5. Jesus' ascension (Acts 1) occurs before the word "Christian" is invented (Acts 11) making it impossible for Jesus to be a Christian where Christian means something other than Jewish.
6. Christians believe Jesus, who is Jewish, is the head of the church. In addition, the church is established and run by Jews before the word "Christian" is invented...making it just as impossible for the church to be ANYTHING but a JEWISH institution according to the New Testament.
While others have the right to claim that after the NT, Christians and Christianity became something other than Jewish, I have the right to maintain the six points above while demanding proof of the post NT event where this change was made. In addition, I also have the right to state that those post NT events do not represent true Christianity because true Christianity was established by the NT...not the changes that occurred later.
So when a person asks the question, ***A Christian dating a Messianic Jew who believes in Christ - good or a disaster waiting to happen?*** The Biblical answer is where the Christian believes in Jesus and the NT like the Messianic Jews who make it up and all but one author (Luke) wrote it.
And to those who state that writings after the Jewish scriptures (which some call the Old Testament) ended aren't to be allowed, then by that law, all the Talmud and the rabbis that wrote it shouldn't be allowed.
At the heart of the question about Christians dating Jews is the question of who is a Jew. Logical and educated answers NEVER define a word by what it IS NOT as the trillions of things it is not can never be fully stated and the answer becomes useless.
Only answers that define what Judaism is should be considered.
If you are going to define who is Jewish by the Jewish scriptures, then that definition has to be applied to all. With any definition pulled out from the Jewish, it must be questioned why that definition is correct given the prophets weren't inspired by HaShem to create that definition.
Second, the Torah itself demands a fair application of the definition. There is not to be one definition for non-Jews and another definition for Jews. If you attempt to apply a definition pulled from the Jewish scriptures, it would be right to question if anyone is actually Jewish today according to that definition.
1. Most people who identify themselves as Jewish don't believe in God or the Torah. Therefore, any definition where the Jewish scriptures were used to set the standard of who is a Jew would be rejected by most Jews.
2. The Judaism of today is radically different than the Judaism of Jewish scriptures.
a. The Jewish scriptures describe Jews practicing Judaism as a people who
1. Follow God and His prophet...meaning that God answers people's prayers and gives them instructions on how to live in real time. Actual words of God are available at the time they are to be put into effect. Those who follow the words given by God are His people...hence Jews.
2. Not following God's instructions is universal. Even Jews don't follow God's instruction so this can not be what determines who is a Jew. Only those who repent and follow God's instructions can qualify as being Jewish.
3. The sacrificial system is part of the Jewish scriptures.
a. The sacrificial system was started outside of Israel and therefore, outside the Temple. The Torah was written outside the Temple and Moses who is credited as writing the Torah made sacrifices in Israel. To suggest that the Torah demands the sacrificial system to only have sacrifices in the Temple is insane.
b. Elijah made a sacrifice outside the Temple while there was a king of Israel on the throne, Israel was at peace with its neighbors, and there was Temple in Jerusalem. This sacrifice
1. was instructed by God
2. ended the severe drought the scriptures describe as caused by God.
3. God Himself added the flame and consumed Elijah's sacrifice on Mount Carmel.
4. The above action by God occurred as a direct answer to prayer.
5. Every Jew present had this personal encounter with God and responded because of their experience with their God.
c. It must be recognized that God can still command His people to make sacrifices outside of Israel as the Torah describes and outside the Temple as 1 King 18 describes. In other words, there is nothing in the Jewish scriptures preventing from God from giving this instruction.
d. It was Rome that destroyed the Temple. If what Rome did is going to be compared with what the Babylonians did, then the Temple should have been rebuilt after the fall of Rome as it was after the fall of the Babylonians.
If the door is opened where Christianity's separation from the NT is questioned, then the door should be open to discuss the separation from Judaism from Jewish scriptures. What ever definition is applied to determine a Christian is not a Jew also needs to be applied to all Jews because the majority will also fail the test.
So when talking about the marriage of Christians and Jews, it is mandatory to define who is a Jew. If not, the conversation is dribble based on personal opinion with nothing based on Jewish scriptures or the NT.