Shimon, the maharal, rizhiner rebbe, and many others had a yichus mesorah as being descendents of Dovid hamelech….that part isn’t the difficulty with the lubavitcher rebbe; the main issue, as you said, is that a “second coming” ideology wherein a Messianic candidate who fails to accomplish his mission must return after death to finish the job.
Let’s go through some criteria: rambam in melachim 12:3 says:
כשתתיישב ממלכתו ויתקבצו אליו כל ישראל. יתייחסו כולם על פיו ברוח הקודש שתנוח עליו. שנאמר {וישב מצרף ומטהר} וגו’. ובני לוי מטהר תחילה ואומר זה מיוחס כהן וזה מיוחס לוי
Moshiach will use ruach hakodesh to identify the yichus of the shvatim, starting with shevet levi – he will clarify who is a cohen, Levi, etc. “All of yisroel will gather unto him”.
Also in melachim 11:3:
ואל יעלה על דעתך שהמלך המשיח צריך לעשות אותות ומופתים ומחדש דברים בעולם או מחיה מתים וכיוצא בדברים אלו. אין הדבר כך. שהרי רבי עקיבא חכם גדול מחכמי משנה היה. והוא היה נושא כליו של בן כוזיבא המלך. והוא היה אומר עליו שהוא המלך המשיח. ודימה הוא וכל חכמי דורו שהוא המלך המשיח. עד שנהרג בעונות. כיון שנהרג נודע להם שאינו
Here the rambam is clearly saying that moshiach need not do miracles, which goes against a lot of the messianic claims that chabad makes – they say that while he accomplished nothing of the following requirements (see below), he was a baal mofes and therefore a candidate for moshiach.
ואם יעמוד מלך מבית דוד הוגה בתורה ועוסק במצות כדוד אביו. כפי תורה שבכתב ושבעל פה. ויכוף כל ישראל לילך בה ולחזק בדקה. וילחם מלחמות ה’. הרי זה בחזקת שהוא משיח. אם עשה והצליח [ וניצח כל האומות שסביביו ] ובנה מקדש במקומו. וקבץ נדחי ישראל הרי זה משיח בודאי
Here the rambam sets up two tiers – one is “chezkas moshiach”, the presumptive moshiach, and the other is “moshiach vadai”, a certain moshiach. If a KING arises from bais dovid, gets ALL of klal yisroel to keep the Torah, and fights the WARS of Hashem, he is assumed to be moshiach. If he returns all of the Jews to eretz yisroel and BUILDS the beis hamikdash, he is definitely moshiach.
There are 13 million jews in the world; 15-18% identify as some sort of orthodox. 94% of American jews are not orthodox. The lubavitcher rebbe accomplished a lot in kiruv – every neshoma is precious – but realistically, only a small fraction of the jewish nation “gathered unto him”, and even when they did, he did not perform this task. Among orthodox jews, the lubavitcher rebbe was one of many influences – rabbi yoshe ber Soloveitchik, rav moshe feinstein, rav aharon kotler, rav Shraga feivel mendelowitz, the satmar rov, the tzelemer rov, rav yosef breuer, the bluzhever rebbe, rav yonasan shteif, the kloizenberger rebbe, and many others all were extremely influential in building American jewry. The fact that the lubavitcher rebbe made headlines sometimes and appeared on television (which chabad makes a huge deal out of for some reason) did not mean that he represented all of klal yisroel. He exerted himself to help russian jewry, but sadly the overwhelming majority remains completely disenfranchised from yiddishkeit, 27 uears after his passing.
They claim that he “fought the wars of Hashem” by doing kiruv. That does not fit with the seder of the rambam, because the following words are “if he succeeds and fights all the surrounding nations”
Criteria:
1. Appointment as king (most of the Jewish people did not accept him as their rebbe, let alone king)
2. Getting all of klal yisroel to follow the Torah
3. Fighting the wars of Hashem
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4. Returning HIMSELF all the jews to eretz yisroel, not merely watching the zionists try to do it
5. Building the beis hamikdash bemkomo
The lubavitcher rebbe accomplished none of these whatsoever.