The Minchat Chinukh, in discussing the mitzva to build a Beit Ha-mikdash (95), takes note of an intriguing omission in the Rambam’s presentation of the laws relevant to the Mikdash and its various components. In Hilkhot Beit Ha-bechira, the Rambam presents the details of the construction of the Mikdash and the keilim (appurtenances), with one glaring exception – the aron (ark). Whereas the Torah in Parashat Teruma discusses the laws regarding the construction of the ark along with the rest of the laws concerning the construction of the Mishkan (Shemot 25:10-22), the Rambam does not discuss the aron in his presentation of these laws. The Minchat Chinukh concluded on this basis that in the Rambam’s view, there is no eternal command to build an aron. Whereas the commands in the Torah regarding the other keilim indeed apply for all time, as they must be constructed whenever circumstances allow for building a Mikdash, the command to build an aron does not apply for all time. Indeed, the Second Temple functioned for hundreds of years without an aron. (The Gemara in Masekhet Yoma (53b) cites a debate as to whether the ark was buried underneath the Temple towards the end of the First Commonwealth, or if it was brought to Babylonia after the destruction. All agree, however, that the aron was not present in the Second Temple.) The Minchat Chinukh leaves it as an unanswered question why the command to build an aron is not regarded as an eternal command like the commands to build the other keilim.
Rav David Rappaport, in his Mikdash David (2:1), suggests an answer based on the Gemara’s comment in Masekhet Shavuot (15a) regarding the method whereby the keilim in the Beit Ha-mikdash became formally consecrated. The Gemara establishes that the initial keilim build in the wilderness in the time of Moshe became formally consecrated by being anointing with the shemen ha-mishcha (anointing oil). Later in Sefer Shemot (30:22-33), we read of the command to prepare this special oil with which the Mishkan, its appurtenances, and the kohanim were consecrated. The Gemara comments that this method of consecration was required for the keilim built at that time, at Sinai, but not for keilim constructed henceforth. In the future, whenever it became necessary to build a new altar or menorah, for example, the given article became formally consecrated by being used for the first time. Using the newly constructed article for its intended purpose conferred upon it a halakhic status of sanctity, in lieu of anointing with the shemen ha-mishcha.
On this basis, Rav Rappaport explained, we can easily understand why the Rambam does not recognize an eternal mitzva to construct an ark. The ark is unique among all the keilim in the Beit Ha-mikdash in that it is not needed for any ritual. Although sacrificial blood is sprinkled on the aron as part of the Yom Kippur service, the Gemara establishes explicitly in Masekhet Menachot (27b) that the blood needs to be sprinkled towards the site of the aron, regardless of whether the aron is actually present. And for this reason, the Gemara comments, the Yom Kippur service was performed even in the Second Temple, even though it did not contain the ark. As such, it was inherently impossible to ever use another aron. Since the only way to consecrate the keilim is to use them for their intended purpose, there is no possible method of consecrating the aron, which was not needed for any ritual in the Mikdash. For this reason, Rav Rappaport speculates, the Rambam worked off the assumption that no aron would ever be built after the original aron built in the wilderness, and so he did not include the laws of constructing the aron along the laws of the Mikdash and its appurtenances.
However, Rav Rappaport then notes that this approach depends on the question as to whether a formal avoda (service in the Mikdash) is necessary for the consecration of the keilim. The Gemara’s ruling could be understood to mean simply that the article in question needs to be used in the way it is intended to be used. This does not necessarily require the performance of a formal avoda. If so, then a new aron could be consecrated simply by being placed in its assigned location in the Beit Ha-mikdash. This consideration, then, would not be a reason for the Rambam to omit the construction of the aron from his discussion of the construction of the Temple and its furnishings.
Some have suggested that the source for the Rambam’s view – that there is no eternal command to construct an aron – is the opinion noted earlier that the aron was hidden beneath the Temple before the Babylonian conquest of Jerusalem. The Rambam accepts this position, and writes in Hilkhot Beit Ha-bechira (4:1) that King Shlomo constructed the First Temple with underground vaults in which the aron would be stored and hidden when necessary. Notably, this was done only for the ark, and not for any of the Temple’s other furnishings. Apparently, it was understood that the original ark constructed in the times of Moshe in the wilderness was the only aron that would ever be built, and it thus had to be hidden as the specter of the Temple’s destruction loomed. This would prove that there is no eternal command to construct an aron, as the aron built in the wilderness was destined to be the only one ever constructed.