Reply To: Bein Hametzarim Trivia Question

@GefilteFish,

Excellent question.

The Lubavitcher Rebbe discusses this (חידושים וביאורים בהלכות מלכים סי’ ב, לקוטי שיחות חכ”ז ע’ 191 ואילך), here is a synopsis which I wrote elsewhere:

The Rambam rules that the third Beis Hamikdash will be built (by humans, and specifically) through Moshiach, in order to fulfill the mitzvah of building the Beis Hamikdash. This opinion is also brought in Chazal.
(ראה ירושלמי מגילה פ”א הי”א. פסחים פ”ט ה”א לגי’ המנ”ח מצוה ש”פ. ויק”ר פ”ט, ו. במדב”ר פי”ג, ב)

Others hold that the third Beis Hamikdash was already built in heaven and will descend at the time of Geula.
(רש”י סוכה מא, סע”א. ר”ה ל, סע”א. תוספות סוכה שם. שבועות טו, ב. תנחומא פקודי יא. זח”א כח,א. ועוד)

We can reconcile both opinions in several ways:

1) The physical Beis Hamikdash will be built by Moshiach, and the “spiritual Beis Hamikdash” of fire will descend from heaven and enclothe itself in the physical building. Similar to the fire on the mizbeach which was ignited from a physical flame, and a spiritual flame also descended from above.
(ערוך לנר סוכה שם ד”ה א”נ)

2) The Midrash explains that the gates of the Beis Hamikdash sunk into the ground and were buried in Har Habayis. The third Beis Hamikdash will descend from heaven, and the gates will rise up from the ground, and will be set into place by the Yidden. Halachically, erecting doors is considered erecting the entire structure, so the Yidden will thus fulfill the mitzvah of building the entire Beis Hamikdash.
(איכה ב, ט: “טבעו בארץ שערי'”. במדב”ר פט”ו, יג, ועוד. ב”ב נג, ב)

3) The details of the Beis Hamikdash which are already known to us (from מסכת מדות) will be built by Moshiach. The unknown parts which are described cryptically in nevuas Yechezkel will descend from heaven.

4) If we are worthy of the Geula (“זכו”) – the Beis Hamikdash will descend from heaven. If the Geula comes at its set date without us being worthy (“לא זכו”) – we will have to build the Beis Hamikdash ourselves with Moshiach.
(ע”פ הגמרא בסנהדרין צח, א שבמצב של זכו הגאולה באה באופן נסי, ובמצב של לא זכו באה ע”פ טבע)

The reason why the Rambam only mentions the human aspect of the Beis Hamikdash is because his focus throughout Mishneh Torah is on human behavior (“ספר הלכות”). Additionally, the Rambam only describes the Geula in a “לא זכו” fashion, although it seems that he agrees with the possibility of זכו.
(אגרת תחיית המתים ס”ו: אין דברנו זה החלטי)

@GefilteFish, according to options 2 and 4, the Rambam agrees that the Beis Hamikdash can/will descend from heaven which fits with the Gemara Rosh Hashana.

@Reb Eliezer, I would love to see some input.

מנחם שמו