To teach us that the spiritual content of what needs to be inside the Mikdosh needs to be inside each and every one of us.
Sanctuary: Which has the primary resting of the Shechina of the world. Which is a general aspect of the Mikdosh. Which shall also be inside every person.
Details of the Beis HaMikdosh: Besides building a sanctuary, the other vessels of the Beis HaMikdosh shall also have representation in your heart. For example, the Mizbeiach can represent the idea of sacrificing all one’s over-indulging akin to the fats that were sacrificed on the alter. The menorah can represent the idea of illuminating the world and doing acts of goodness and kindness for others. The spices can represent the concept of pure thoughts. The washbasin can represent the preparation and for and the beautification of a mitzvah. The ark one’s Torah study. The showbread one’s inviting guests and feeding the poor etc.
Several places: In Chassidus (as can be seen at the end of this publication at the Hebrew footnote 18 for a list of places).
Sefer HaMitzvohs: Widely referred to as ‘Derech Mitzvosechoh’, in the Maamor which discusses the Mitzvah of building the Beis HaMikdosh.
