Rebuilding the Bais Hamikdash
In those days, when the Bais Hamikdash stood in its place, the physical building from wood and stone was also a symbol of the spiritual Bais Hamikdash of and within the Jewish people. The physical Bais Hamikdash was able to exist only as long as the spiritual Bais Hamikdash was in existence.
When the first Bais Hamikdash was destroyed -- the spiritual [structure] by the Jewish people itself and the physical [structure] immediately afterwards by Nevuchadnezzar King of Bavel -- it generated for our people a double mission: first, to erect anew the spiritual Bais Hamikdash, and then the physical building, the second Temple. Similarly, the second Bais Hamikdash survived so long its spiritual component was in existence, and with the destruction of its spiritual component, so, too, the physical component of the Bais Hamikdash -- in a way that could not be prevented -- was destroyed.
Regarding the 3rd Bais Hamikdash, the Almighty instructs us that our primary mission -- and according to many opinions, the only one -- is to rebuild the spiritual part alone. The Almighty Himself will take care of the physical part. According to the opinion of Rashi, Tosfos, and others, the 3rd Bais Hamikdash will be built by the hands of heaven.
Therefore, the spiritual Bais Hamikdash must be built through our efforts, through tshuva, Torah, and mitzvos. Every spiritual "brick" that a Jew forms is indestructible, and the process of laying these spiritual "bricks" came about at the same moment that the last part of the 2nd Bais Hamikdash went up in flames. This is one of the deeper meanings of the story in the midrash (Eicha Rabba) that at the moment the Bais Hamikdash was destroyed, Moshiach was born.
The main parts of rebuilding the spiritual Bais Hamikdash which Jews need to build in this long, dark golus, were already built by the earlier generations of spiritual giants of our people. What remains for us is only to complete a small part. But our part must be permeated with the same kedusha of the earlier generations.
The conclusion from this is: that we ourselves should be permeated with this idea, that every good action according to Torah that we do in our day-to-day life, as much as this action may seem large of small to our fleshly eyes, we are thus laying one of the last "bricks" of the eternal Bais Hamikdash for our people and for the world in general, and thus he is building his part of the spiritual Bais Hamikdash and the physical Bais Hamikdash will be built immediately afterward, speedily in our day.
May the Almighty grant that we will merit to the building of the Bais Hamikdash, the physical one also, speedily in our days, amen.
With wishes for all good,
HaRav Menachem Schneerson
Personal letter of the Rebbe, 13 Menachem Av, 5709 (1949), printed in Igros Kodesh v.3, p.164