Part of Why Legalistic "Messianic Judaism" Is Not Jewish (Or Messianic Jewish)

Looking at Parshot Terumah and Tetzaveh, at least for me, the following comes to mind: "Some institutions are considered to be a product of the cultural milieu and societal norms of the ancient Near East when the Hebrew Scriptures were written down, and do not speak to our lives today." (Amy Scheinerman, URJ.org) While they do not lessen my belief in the inerrant and infallible Word of G-d, they do point out that "Some institutions... do not speak to our lives today" but for that they were instituted when chen (grace) was not habrit l'Yisra'el. 

Besides, the Mishkan (which took a lot of time, effort, and resources to build) and HaBeit Mikdash are both gone. So, how can one possibly expect to keep all 613 mitzvot, some of which relate to the long-past Mishkan and the yet-to-be-rebuilt Beit Mikdash?