The Yalkut Shimoni (a midrash) on the Book of First Samuel (106) teaches us an awesome lesson about the Churban/destruction and exile by the Romans, Rabbi Simeon Bar Menasya said, “Israel was only exiled after it rejected the following three things, Malchut Shamayim/the Kingdom of Heaven, Malchut Beit David/Kingship from the House of David, and the Beit HaMikdash/the Holy Temple.
So we learn from this, that the cause of the exile of the Jewish people, was because Jews took too lightly, Torah observance (God’s rule), we should have demanded a rightful Davidic descendant be appointed as king, once the Maccabees drove out the Syrian Greeks, and Jews didn’t care enough about the Holy Temple in Jerusalem.
A variant text in the Yalkut Shimoni explains from a different angle, why the Roman exile would be so long, as opposed to the limited seventy years of the Babylonian exile, after the destruction of the First Temple. Rabbi Simeon Bar Menasya predicted, “In the future the following three things will be despised by Israel, the Kingdom of Heaven, David’s kingdom [the Messiah], and the building of the [Third] Temple.”
By the attitudes and behavior of most Jews today; the rampant secularization and assimilation; apathy about the Messianic concept and the Temple; Rabbi Simeon’s prediction seems to have come true.
But there is a third variant text, that gives the solution, how to end the exile. Rabbi Simeon Bar Menasya said, “Israel will not see a sign of the redemption until they [the Jews] repent and demand the following three things, the Kingdom of Heaven, David’s kingdom [the Messiah], and the building of the [Third] Temple.”
There are many things to do everyday, like saying the section about Korbanot (Offerings), written in the prayer book, at the beginning of the Shacharit (morning) and Mincha (afternoon) prayers.
And saying Tikkun Chatzot, the midnight prayer, mourning, the exile of the Jewish people, the lack of honor paid to the God of Israel by the nations of the world, and the feeling of lack, that we no longer have the Beit HaMikdash.
Get any one of a number of books about the Beit HaMikdash, available to buy these days (many with charts, diagrams, and pictures). Learn about the temple, the priesthood, and sacrificial system, their halachic details, and deeper mystical meanings.
Then if you’re in Israel, there’s Aliyah to Har HaBayit (going up to the Temple Mount within the known permitted areas), to show to the Arabs and the world, a Jewish presence there, and to God, that the Jewish people still care.
And there is also working with, or donating money to, any of a number of temple organizations and yeshivot (rabbinical colleges), working to construct vessels for future usage, learning all the pertinent laws of the temple, priesthood, and sacrificial system, promoting Jewish rights on the Temple Mount, and protecting the area of the Temple Mount from destruction of artifacts and Arabization, etc.
These increased activities, are all in addition to saying every Bracha/Blessing and every prayer, doing every mitzvah and learning Torah, with the intention of increasing God's Honor, the honor of Israel, the thought of bringing the messiah, and helping to bring about the rebuilding of the Holy Temple.
But by far the simplest thing to do, something everybody could do, is simply to ask yourself everyday, “What have I done for the Beit HaMikdash today?”
If you think that all these activities are a waste of time, that God will send the Messiah to build the Third Temple when He feels like it and nothing we do will help, think again!
“R. Acha said: This teaches that the Holy Temple will be built before the throne of the house of David is established,” (Talmud Yerushalmi Maaser Sheni 5:2, Vilna 29b).
See also, Tosefot Yom Tov, Rashash, Malechet Shlomo, Ibid. Shnei Luchot HaBrit, Beit David 1:376. Cf. Megillah 17b end. In Yalkut 2:499, we find that the Messiah will reveal himself on the Temple roof, i.e. the Holy Temple is already built. See also VaYoel Moshe 55f (from the Satmar Rebbe), and Rabbi Yehuda Gershoni, Mishpat HaMelucha 11:1.
And, take a look at Rabbi Aryeh Leib ben Asher Gunzburg (c. 1695 – 1785), also known as the Shaagas Aryeh, a Lithuanian rabbi, and author of the Gevurot Ari, on Ta’anit 17. There it states, that certain Kohanim/priests are forbidden all year round to drink wine, from the Gemara Rosh HaShanah 11a, which cites a dispute between R. Eliezer and R. Yehoshua whether the redemption is going to happen in Nissan or Tishri.
The Gevurot Ari asks on the Gemara there, a question, it should be sufficient to ban drinking wine only for these two months, so why is it necessary to ban it for the entire year?
He answers on the basis of the Talmud Yerushalmi Maaser Sheni 5:2 that the Beit HaMikdash will be built before the Messiah from the House of David comes. He writes that according to this there is no fixed day for the building of the Beit HaMikdash – it could be built any day of the year. In contrast he explains that, R. Eliezer and R. Yehoshua discuss when the Messiah will come, which can only be in these months.
So, in fact, Rabbi Simeon was telling the Jewish people, to be redeemed, we need to implement these Mitzvot/policies that will re-build the ideal national structure in the Land of Israel, including building the Third Temple.