This week's haftarah - read only during a leap year, since in a regular year, Tetzave coincides with Parashat Zachor, and a special haftarah is read - speaks of Yechezkel's vision of the third Beis HaMikdash. Even before the destruction of the second Temple, Yechezkel gave B'nei Yisrael reason to feel shame over their sins and inspire them to repentance with hope for the future by prophesying to them about the glory of the ultimate meeting place of Hashem and His people. The fact that a detailed blueprint is presented seems to indicate that this structure will be built by the Jewish people. Rashi (Sukkah 41a) quotes the Midrash Chazal that the third temple will be lowered fully complete from the heavens. This approach will have to assume that the blueprint was not written down by Yechezkel to serve as a building plan but rather as a description of its splendor. Rambam (Hilchot M'lachim 11:1,4), though, states that the Melech HaMashiach will build the Beis HaMikdash and that this will be one of the means of verifying him as the messiah. The fact that the Rambam codifies the structure of the second Beis HaMikdash as recorded in Maseches Middos (see his commentary to the beginning of Middos and Hilchos Beis HaBechira 1:4) also verifies that his view is that the third Beis HaMidkash will be built by man.
Rav Yaakov Etlinger, in his commentary Aruch LaNeir (Sukkah 41a), questions Rashi's interpretation of this tradition that the Mikdash will descend from heaven. Every day we pray "sheyibaneh beis hamikdah" - that the Temple should be built speedily in our days. We do not say that it should be revealed but that it should be built. Based on this and other difficulties with Rashi's interpretation, the Aruch LaNeir concludes that the third Beis HaMikdash will be built by man, and then, the heavenly Beis HaMikdash will enter it and fill it with sanctity just as the soul gives life to the body. (See there for his resolution of the Gemara which led Rashi to assume that the Beis HaMikdash will descend fully built.)[2]
This debate perhaps is dependent on two different interpretations of the Gemara in Megilla (17b) which describes the order of the Shemone Esrei. There, the Talmud assumes that the blessings from "Bareich aleinu" until the end of the Shemone Esrei describe the order of the final redemption. The Gemara states that the righteous - referred to by the blessing of "'Al hatzaddikim" - will be elevated in Yerushalayim; hence, the blessing of "Bonei Yerushalayim" is recited immediately after the blessing of "'Al hatzaddikim". Rashi (18a s.v. "v'achar yashuvu") assumes that this last blessing refers to the rebuilding of the Beis HaMikdash. This interpretation is verified by the phrase "v'tishkon b'tocha" - "and dwell in it" - which we recite in the blessing, a reference to the dwelling of the Shechina in the Mikdash. The insertion of nacheim on Tish'a B'Av which refers to the rebuilding of Jerusalem with fire is also a reference to the rebuilding of the Beis HaMikdash (see Bava Kamma 60b).[3] However, Maharsha interprets that later blessings refer to the building of the Beis HaMikdash. The Gemara states that "'Es tzemach david" of course refers to the coming of Mashiach ben David, followed by "Shema Koleinu" and "Retzei" which refer to the tefila and korbanos respectively to be offered in the third Beis HaMikdash. Since the order of mitzvot upon entry into Eretz Yisrael is first the appointment of a king and only then the building of the Mikdash (see Rambam Hilchos M'lachim 1:1), the Shemone Esrei reflects this order. According to Maharsha, "Bonei Yerushalayim" refers to the rebuilding and settlement of the physical city. Rashi's interpretation of this Gemara leads to the conclusion that the Beis HaMikdash will be built before Mashiach ben David appears. These two interpretations seem to reflect the two approaches mentioned above. If the Temple precedes the coming of Mashiach, it presumably will be built by the Jewish people.[4] If, however, it succeeds the coming of Mashiach, it might descend from the heavens.
Perhaps we can resolve the difficulty of the Aruch LaNeir on Rashi's interpretation of the Midrash from our prayers that the Beis HaMikdash be built. There is a commonly stated idea that each mitzva we perform adds a "brick" to the heavenly Temple. [Admittedly, I am unaware of the source for this notion. Perhaps this is based on the Gemara (B'rachos 6b) that he who gladdens a Chassan and Kalla is as if he rebuilt one of the churvot Yerushalayim. The popular notion seems to extend this to all mitzvos.] Therefore, we ask Hashem that the heavenly Beis HaMikdash should be rebuilt in our days, namely, that we merit the performance of all of the necessary mitzvot, and subsequently merit its descent. May the haftarah describing the glory of the third and final Mikdash, and, more importantly, what it represents, the state of intense connection to our Creator, inspire us to dedicate ourselves to the service of HaKadosh Baruch Hu with ever greater enthusiasm.
[1] See also Beit HaMikdash: Built by Whom? Here, we analyze the topic with several different sources.
[2] Also see D'rishas Tzion by Rav Tzi Hirsch Kalischer for a presentation of many sources that the Mikdash will be built by the Jewish people.
[3] Rav Y. D. Soloveitchik zt"l, along with most Gedolei Yisrael, insisted that the nusach of nacheim, even the reference to Yerushalayim as being utterly destroyed, disgraced, and desolate, etc. not be modified at all, even after we merited the physical rebuilding of the city. As long as the Mikdash is not rebuilt, Yerushalayim, by definition, is b'churbana - in a state of destruction. However beautiful the physical city of Jerusalem is, it is like a lifeless body without a soul. [See B'Ikvei HaTzon (18:10) by mori v'Rabi Rav H. Schachter shlit"a.]
[4] This suggestion, though, would make Rashi to Megilla (17b) inconsistent with his comments to Sukkah (41a).