Rav Wolbe (Shiurei Chumash) sites the Sforno who explains the commandment to build the Mishkan. Before the sin of the golden calf, Bnei Yisroel were able to build a mizbeiach anywhere at all. Hashem's glory filled the world and, therefore, one was allowed to offer a sacrifice wherever he wished, as the pasuk states, "Build for Me a mizbeiach . . . any place that you mention My Name I will come to you and bless you" (Shemos 20, 21). This was not the case after they sinned with the golden calf. Hashem decided that His glory would have to be concentrated in a specific place. Hence, we were commanded to build the Mishkan to serve as that abode, and from then on sacrifices could only be offered in the Mishkan or Beis Hamikdosh.
Rav Wolbe elaborates, that the Mishkan was to act as a virtual world; a microcosm of the entire universe. As Chazal tell us, the hooks that held the yerios (coverings of theMishkan)together twinkled like the stars, the beams of the Mishkan paralleled the angels on high, and each of the utensils mirrored a different aspect of the universe. Originally, the world was created pure, but became tainted shortly thereafter when Adam sinned by eating from the eitz hadas. When Bnei Yisroel heard the Aseres Hadibros at Har Sinai, they regained the purity of Adam before he sinned. However, they lost that status with the sin of the golden calf. The Mishkan was intended to fill the void and act as a miniature world devoid of sin.
It was for this reason that all those who were ritually impure were not permitted to enter the Mishkan. Ritual impurity associated with death comes as a result of sins, for as Chazal tell us, "It is not [the bite of] the snake that causes death, rather, it is the sin that causes death." If there were no sin, there would be no deaths. The Kuzari explains how all the ritual impurities are in some way connected to death. Sins, and therefore death, have no place in this newfound utopian world. In this abode, Hashem's Shechina could reside.
Chazal also explain the commandment to build the Mishkan almost homiletically. "Build for Me a Mikdash, and I will dwell within - each and every one of you." If the world around us is filled with decadence, each of us must attempt to create an environment that resembles the Mishkan. We must ensure that the enticements and sins of the secular world do not enter into our homes thereby enabling it to act as an oasis for Hashem's Shechina.