The outer mizbei’ach, the altar that stood in the courtyard of the Mishkan, was coated with a layer of copper. Rashi explains that copper is a metaphor for brazenness (see Yeshayahu 48:4), and likewise the mizbei’ach atoned for Bnei Yisrael’s brazen and insolent behavior.
There is also a positive form of brazenness that the copper coating of themizbei’ach represents. Namely, this means that a Jew must be bold and unashamed about his Judaism, defiantly unfazed by those who might mock his worship of G-d.
Beneath its external toughness, however, the inside of the mizbei’ach was hollow, and in each location that Bnei Yisrael assembled the Mishkan, themizbei’ach would be filled with earth. This symbolizes that internally we must always feel humble, like the insignificant and trampled-upon earth—even while approaching any challenge to Judaism with toughness and chutzpah.
These two aspects in the construction of the mizbei’ach, its copper coating and its hollow inside filled with earth, represent two contradictory, yet vital qualities that a Jew must always have.