Testimony draws down the Shechinah

גם פי' משכן העדות שזה המשכן נעשה ע"י עדות בנ"י כמו שכתוב ואתם עדי כו' ואני אל כי בנ"י בכח זה העדות יכולין להמשיך השכינה בעולם כדכתיב על פי שני עדים יקום דבר.

Also the meaning of "Mishkan ha-Eidus" (the Tabernacle of Testimony): this Mishkan was made through the testimony of Bnei Yisrael, as it is written, "And you are My witnesses… and I am God" — for through the power of this testimony Bnei Yisrael are able to draw the Shechinah into the world, as it is written, "By the word of two witnesses a matter shall be established."

The Sefas Emes explains why the Mishkan is called the "Tabernacle of Testimony": it stands as a witness, and it is built through the testimony of Bnei Yisrael themselves. When Bnei Yisrael testify to Hashem's presence ("you are My witnesses, and I am God"), that testimony has the power to draw the Shechinah down into the world — for just as two witnesses establish a legal matter, the people's witnessing establishes the Divine Presence below.

ק"ש בשחרית ובערבית.

Krias Shema in the morning and in the evening.

These are the "two witnesses": the twice-daily declaration of Shema, morning and night, by which Bnei Yisrael testify to Hashem's oneness and kingship.

ולפי שהי' דור המדבר עדים נאמנים כמ"ש אתה הראת לדעת.

And because the generation of the wilderness were faithful witnesses, as it is said, "You have been shown, to know…"

The generation that stood at Har Sinai personally beheld Hashem's revelation ("you have been shown, to know that Hashem is God") and were therefore reliable witnesses to His presence.

זכו להשראת השכינה במשכן וכמו כן במקדש.

They merited the resting of the Shechinah in the Mishkan, and likewise in the Mikdash.

Because of their faithful testimony, they were worthy that the Shechinah dwell among them in the Mishkan, and the same held true later for the Bais Hamikdash.

ונמצא המשכן עדות על ישראל שהם נאמנים בעדותן.

It emerges that the Mishkan is testimony about Yisrael — that they are faithful in their testimony.

The relationship runs both ways: the Mishkan itself testifies that Bnei Yisrael are trustworthy witnesses to Hashem, for the Shechinah rests only where there is faithful testimony.

ולכן עתה בעוה"ר שמקדש חרב אמרו עדיכם בירה.

Therefore now, in our great sorrow, when the Mikdash is destroyed, they said: "Your witness is the Temple."

Chazal taught that "your witness is the Bira (the Temple)" — the very existence (or absence) of the Bais Hamikdash testifies to the state of Bnei Yisrael's relationship with Hashem. In galus, with the Mikdash destroyed, that testimony is, tragically, lacking.

שאם היינו מקבלין עול מלכות שמים בכל יום כראוי במסירת נפש היו זוכין לבנין המקדש.

For if we would accept upon ourselves the yoke of malchus Shomayim each day as is fitting, with mesiras nefesh, we would merit the rebuilding of the Mikdash.

Were we to truly take on the ol malchus Shomayim every day — with the self-sacrifice that the Shema demands — our daily testimony would once again be strong enough to draw down the Shechinah and rebuild the Bais Hamikdash.

כי המקדש תלוי בעדות ישראל.

For the Mikdash depends on the testimony of Yisrael.

The standing of the Bais Hamikdash hinges entirely on the faithfulness of Bnei Yisrael's testimony to Hashem.

וזה משכן העדות:

And this is the meaning of "Mishkan ha-Eidus" (the Tabernacle of Testimony).

This, then, is why it is called the Tabernacle of Testimony: it both rests upon, and bears witness to, the faithful testimony of Bnei Yisrael.

Summary: The Sefas Emes explains that the Mishkan is called the "Tabernacle of Testimony" because it is built upon, and itself testifies to, the faithful witness of Bnei Yisrael. Through their twice-daily testimony in Krias Shema — accepting the yoke of malchus Shomayim with mesiras nefesh — Bnei Yisrael draw the Shechinah into the world. The Bais Hamikdash depends on this testimony; were we to renew it properly each day, we would merit its rebuilding.