It is written in Zohar that the Holy Temple will be built by G-d Himself. Similarly, Rashi writes, "The final Temple that we are awaiting, is built and complete, and will be revealed and descend from heaven."

On the other hand, Maimonides assumes that the Temple will be built by the Jewish people, led by Moshiach.  Presumably, his opinion is based on the verse "And they will make for Me a Sanctuary and I will dwell in their midst."  G-d gave us a commandment to build a Holy Temple for Him, and the obligation rests upon us to complete it.

One way of reconciling the two opinions is as follows:  The building, which was built by G-d, will descend from heaven, and the doors and gates will be affixed by the Jewish people. 

During the destruction of the second Temple, the gates sunk into the earth and were hidden, as the verse states, "Her gates sunk into the ground."  When the Redemption will come, the Sanctuary will descend from heaven, and at the same time, the gates and doors hidden in the earth will rise.  Then the Jewish people will affix them in their proper places.

Placing the doors is a significant stage in building the Sanctuary, as the doors make the building complete and fit for habitation.  In a certain sense, the one who puts the final touches is like the one who built the entire structure.

If so, the final Temple will represent a fusion of G-d's work and the work of human beings.  Because it is G-d's handiwork, it will have permanence. On the other hand, since G-d is loving and merciful, He will give us the opportunity to participate in the building, to give us the merit of constructing a home for Him.