Why is Moshiach a “fundamental belief” (iqar)?

Question

In what sense is belief in the eventual coming of Moshiach fundamental to Judaism? Rambam famously includes it as the twelfth in his list of iqarim, but why?

Considering the other iqarim, Judaism would be very different without them—non-existent or nonsensical without some. But it seems to me that if we had been promised something else in place of a restoration of the Davidic monarchy (a generic “Messianic Age”, or a Torah republic, etc.) there wouldn’t be much difference in Jewish practice or thought.

Note: I’m not asking whether belief in Moshiach is required; I’m assuming that since Moshiach is prophesied, belief in the eventual redemption and restoration of the Davidic monarchy is a necessary consequence of belief in nevu’ah. I’m also not asking why a melekh hamoshiach is needed (rather than the alternatives I mentioned above). I’m only asking why (or, in what sense) this belief is part of Judaism’s foundations rather than being one of its consequences.

Answer 1

It is part of the fundamental principle of yichud Hashem - that not only is there only one God, but God alone has full control of the the world and His will reigns supreme. The Ramchal starts off klach pitchei chachma discussing this. (See also Daat Tevunot).

There he explains that everything in the world, (even what a human being does with free will such as sinning) must be authorized by God. Even when a person commits a sin, it must first be authorized by Him and He considers how it will fit in His divine plan of total rectification in the end. This rectification will be accomplished through either repentance or punishment.

Without this fundamental belief, the Ramchal explains, one can assert that the Jews sinned against God and "ruined" His plan, and are therefore eternally damned (as some Christians believe).

Hence, the belief in Messiah is the belief that history is not a chain of events going nowhere, but there is a plan for creation and that God has exclusive control of that plan and history is inevitably moving towards this goal.

(this last statement by an audio from Rabbi Uziel Milevsky zt'l)

Answer 2

The question assumes that the Ikkar is independent of a generic Messianic age. Rather the Ikkar is about it being fundamental to believe in a Messianic age. The Ikkar is:

1) That the complete fulfillment of Torah and Mitzvos is the purpose of the world. 2) That G-d will ultimately make that happen through Moshiach. 3) To want that to happen.

If someone doesn't think that about Torah and Mitzvos, then they are fundamentally missing something, not really different than someone fully keeping Torah and Mitzvos but not really believing reward and punishment - it takes it that they are of no real consequence.

This is from an article from Rabbi Yoel Kahn in one of his articles from the series called Macheves HaChassidus which was printed regularly in Kfar Chabad Magazine over 20 years ago. I have a binding of a bunch of those articles that was made as a special printing back in the day, and the one I'm referring to is titled למה האמונה בביאת המשיח היא יסוד בדת? on page 132. Part of this book was re-typeset and reprinted in 2001 and is available here, but that is only volume one, which does not cover this article. Perhaps a volume 2 exists or is coming out (the Rabbi is still alive and actively writing).

On the specific question of why Moshiach and not without a king, that is a detail, not unlike why believing in Chabakkuk is a necessary part of the Ikkarim, even though Chabakkuk on his own isn't fundamental to Judaism. Similarly, although Moshiach as a king of specified linage is a specific detail of Messianic Redemption, it is the general concept he is tied to that makes it fundamental.

That last paragraph is not in the article I mentioned. However, he does explore the difference between what is fundamental and what is not. What is not fundamental, if someone mistakenly believes something (in my example above, it would be that there was no such prophet as Chabakkuk) out of ignorance, that is just ignorance, not a lacking in his faith. But if he doesn't believe in an Ikkar, then his faith is bad. (If memory serves, that distinction is from Rav Chaim Brisker).

Answer 3

According to the Hattam Soffer (Responsa vol II YD 356) it is not, to use the OP's words "part of Judaism’s foundations", but rather, is "one of its consequences":

אך א"א לי בשום אופן להאמין שיהי' גאולתינו א' מעיקרי הדת ושאם יפול היסוד תפול החומה חלילה ושנאמר אלו הי' ח"ו חטאנו גורמים שיגרש אותנו גירוש עולם וכדס"ל לר"ע בעשרת השבטים שהם נדחים לעולם המפני זה רשאים הם לפרוק עול מלכות השמים או לשנות קוצו של יו"ד /י'/ אפי' מדברי רבנן חלילה אנחנו לא נעבוד ה' לאכול פרי הארץ ולשבוע מטוב' לעשות רצונך אלקי חפצתי ועכ"פ ועל כל אופן עבדי ד' אנחנו יעשה עמנו כרצונו וחפצו ואין זה עיקר ולא יסוד לבנות עליו שום בנין

However, I cannot believe that our redemption would be one of the principles of our faith, and that if this pillar would fall, then the wall would fall, heaven fore-fend, and that if heaven fore-fend, our sins would cause that He expel us eternally - and as R. Akiva reckoned regarding the ten tribes; that they are eternally rejected. Would this be grounds to reject the yolk of heaven, or to change the dot of an i of even a rabbinic matter. Heaven fore-fend! We will not serve God to eat the fruit of the land and to sate ourselves from its goodness; it is to do your will, my God that I have desired! And in every way and circumstance, we are servants of God, he will do with us according to his will and desire. And this is not a pillar nor a foundation upon which to build any structure. (trans. my own).

Instead, he asserts that its importance stems from the fact that this is a prophecy:

אך כיון שעיקר יסוד הכל להאמין בתורה ובנביאים ושם נאמר גאולתינו האחרונה בפ' נצבים ובפ' האזינו כמ"ש רמב"ן שם והרבה מזה בדברי נביאים אם כן מי שמפקפק על הגאולה הלז הרי כופר בעיקר האמנת התורה והנביאים.

Nevertheless, since the ultimate principle of everything is to believe in the Torah and Prophets, and our final redemption is stated there, in the Parsha of Nizavim, as Ramban stated there, and the words of the prophets include much of this, therefore, one who casts doubt on the redemption denies the principle of belief in the Torah and Prophets. (trans. my own.)

This is a fascinating alternative understanding.