The order of the events which occur in the last six parshiot (portions) in the book of Shemot is an area of controversy among the biblical commentators. Rashi (31:18), citing the principle of "Ain mukdam um'uchar batorah" (the events of the Torah are not presented in chronological order) posits that the commandment to construct the Mishkan occurred after chait ha'egel (the sin of the golden calf). Ramban (35:1) states that the events which are described in the last six parshiot in the book of Shemot actually occurred in the order in which they are presented. He distinguishes between G-d's commandment to Moshe regarding the construction of the Mishkan and Moshe's later commandment to Bnei Yisrael. According to Ramban, Moshe was instructed about the Mishkan prior to chait ha'egel (parshot Terumah and Tetzaveh). Chait ha'egel occurred on the 17th of Tammuz (parshat Ki Tissa). Moshe did not command Bnei Yisrael to construct the Mishkan until after he was assured that G-d had forgiven them for their transgression. Moshe then instructed Bnei Yisrael about the construction of the Mishkan in parshiot Vayakhel and Pekudei.
Rav Shlomo Aviner, in Tal Chermon, notes the differences between G-d's commandment to Moshe regarding the Mishkan and Moshe's commandment to Bnei Yisrael. He states that the differences reflect the discrepancies between the ideal Mishkan and the actual Mishkan which resulted from the chait ha'egel. Regarding the ideal Mishkan (pre-chait ha'egel commandment), G-d commands "from every man you shall take my portion (25:1)." Regarding the actual Mishkan (post-chait ha'egel) Moshe commands, "take from you" (25:5). Rav Aviner suggests that G-d's commandment includes both Bnei Yisrael and the Erev Rav - a large group of Egyptians who joined Bnei Yisrael after witnessing the miracles of the Exodus, whereas Moshe's commandment includes only Bnei Yisrael and excludes the Erev Rav. Moshe's idealistic desire to incorporate this large group into the Jewish people failed. Rather then the Erev Rav being positively influenced by Bnei Yisrael, they actually were instrumental in the chait ha'egel and became a negative influence on Bnei Yisrael. As a result, they were not welcomed into the new, actual Mishkan. Only sincere individuals, rather than large groups of people, would be allowed to join Bnei Yisrael.
A second difference between pre-chait ha'egel and post-chait ha'egel Mishkan invovles the individuals who were to perform the service in the Mishkan. Prior to the sin, all of Bnei Yisrael were instructed, "And you will be a kindgom of Kohanim and a holy nation." (19:6) After the sin, Aharon and his sons were selected to serve G-d on behalf of Bnei Yisrael.
In the Messianic era, the ideal state of service will return, and the impact of the chait ha'egel will disappear. The future Beit Hamikdash will serve as the spiritual center for all the nations of the world. As Jeremiah writes, "At that time people will call Jerusalem 'the Throne of G-d' and all the nations will be gathered to her in the Name of G-d'" (3:17). Additionally, all of Bnei Yisrael will return to the status of Kohanim: "And you will be called the Kohanim of G-d." (Isaiah, 49:6). As a result of our uplifted spiritual state, we will return to the ideal level of closeness to G-d.