Shabbos as a Sign

And you, speak to the children of Israel and say: 'Only keep My Sabbaths! For it is a sign between Me and you for your generations, to know that I, the Lord, make you holy.

וְאַתָּה דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל לֵאמֹר אַךְ אֶת שַׁבְּתֹתַי תִּשְׁמֹרוּ כִּי אוֹת הִוא בֵּינִי וּבֵינֵיכֶם לְדֹרֹתֵיכֶם לָדַעַת כִּי אֲנִי יְ־הֹוָ

Sforno
explains the juxtaposition of the building of the Mishlan next to keeping Shabbos as a hint to us:
If we mess up and damage the "sign", than there is no reason or purpose for a mishkan.


Let's elaborate:

Ramban
The sign acts as sign between us [Hashem and Kllal Yisroel] so that we know:
It says "ushmartem es haShabbos" [Verse 14]
and one who is michallel shabbos [bimeizid bli eidim vihasroah]
is chayiv kareis

Sefer Maor Vashemesh explains:

Midah kineged midah
Shabbos is testimony that there is One Creator.
One who does not keep Shabbos is denying the essence of Unity inherent in One Creator.
Therefore one gets karies which is the ultimate level of pirud, or disunity from He who is One.

The opposite of Dveykus.
Keeping Shabbos allows us, affords us, enables us to
tap into the Unity that is inherent in the Creator of Creation.
It allows us to become One with Hashem.

The Mishkan is a place of becoming one with Hashem.
"Viasu li mikdash vishachanti bisocham"
In them....in us...in all of us...

Opt in for Unity