The word Mishkan is repeated (Rashi, citing the Midrash), alluding to the two Batei Mikdash (replacing the Mishkan) which were taken from us. The word Mishkan has the same letters as the word mashkon, which means collateral. This intimates (say Chazal) that the two Batei Mikdash are collateral for Klal Yisrael’s sins. When we sinned, we lost them, and they are being held in lieu of our repentance, after which the Bais Hamikdash will be restored to its previous glory.
Veritably, in Jewish society, the most important place of worship is one’s own heart. The purpose of the Mishkan’s services was to remind us to live our lives in such a manner that Hashem would be “comfortable,” feel at home with us – in our lives and in our hearts. The Bais Hamikdash replaced the Mishkan as the focus of service. As long as its purpose was being fulfilled, it was untouchable. Once we chose to became apathetic to the vibrancy and centrality of our relationship with Hashem, however, the Bais Hamikdash no longer served a purpose. Its services became meaningless, so that Hashem destroyed the edifice. We destroyed the services; thus, the building was no longer necessary. The Batei Midrash were not destroyed; they simply died when we refused to sustain them through commitment and passion.
Horav Baruch Sorotzkin, zl, posits that actually only one Bais Hamikdash existed. The Bais Hamikdash was the edifice in which Hashem’s Shechinah, Divine Presence, resided. When Klal Yisrael sinned, Hashem removed the Shechinah as collateral until we would do teshuvah, repent, and then be worthy of the return of the Shechinah. The churban, destruction, of the edifice is the collection of collateral. We owe; Hashem collects the mashkon, much like the poor man who is unable to reimburse his debt. His lender takes whatever valuables the borrower has and holds it until that time that the borrower is able to pay his loan. Thus, Hashem collected His collateral twice. There was only one edifice. Hashem took it twice.