Hashem Can Send down the Beis Hamikdash from Heaven, but Our Mitzvah Is to BUILD It!

English follows the Hebrew.

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There is a positive commandment to build a House for The Almighty, prepared for bringing the sacrifices, and for celebrating three times each year, as it says, “Make for me a Temple” (Ex. 25:8); and it is already understood in the Torah the Mishkan (Tabernacle) that Moshe Rabbenu made, in the meantime, as it is said, “For you have not yet come…” (Deut. 12:9) Mishneh Torah, Laws of the Temple 1:1

“For you have not yet come to the rest and to the inheritance, which the LORD your God gives to you. But when you go over the Jordan, and dwell in the land which the LORD your God causes you to inherit, and He gives you rest from all your enemies round about, so that you dwell in safety; then it will come to pass that the place which the LORD your God shall choose to cause His name to dwell there, there you will bring all that I command you: your burnt-offerings, and your sacrifices, your tithes, and the offering of your hand, and all your choice vows which you vow to the LORD.  And you will rejoice before the LORD your God, you, and your sons, and your daughters, and your men-servants, and your maid-servants, and the Levite who is within your gates, for as much as he has no portion nor inheritance with you. Take heed to yourself that you do not offer your burnt-offerings in every place that you see; but rather in the place which the LORD will choose in one of your tribes, there you will offer you burnt-offerings, and there you will do all that I command you.” (Deut. 12:9-14)

“Each generation that it (The Temple) is not built during its time, is considered as if it destroyed it.” Jerusalem Talmud, Yoma 5a (1:1)