Israel as Faithful Witnesses

כתיב אתם עדי כו' עדים אתם כי הקב"ה בחר לו בנ"י להיות מעידין על השי"ת ונתן להם השבת שנק' סהדותא

It is written, "You are My witnesses" (Yeshayah 43:10) — you are witnesses, for the Holy One, Blessed is He, chose Bnei Yisrael to be those who bear testimony concerning Hashem Yisbarach, and He gave them the Shabbos, which is called "sahadusa" (testimony).

Hashem chose Bnei Yisrael to be His witnesses in the world, and the Shabbos itself serves as that testimony, for through keeping Shabbos Bnei Yisrael bear witness that Hashem created the world and rules over it.

וטעם הדברים כי נוגע פסול לעדות ואיתא כי האומות מה שעושין לפעמים קצת טוב הכל לגרמייהו

And the reason for this matter is that one who has a personal stake (a vested interest) is disqualified as a witness; and it is brought down that whatever bit of good the nations of the world sometimes do, it is all for their own sake.

A kosher witness must have no personal stake in the matter; the nations of the world, even when they do some good, are ultimately acting for their own benefit, so they have a negiah (vested interest) that disqualifies them as witnesses.

אבל בנ"י מוסרין נפשם על קדושת שמו ואין כוונתם לעשות המצות על מנת לקבל פרס לתועלת שלהם רק לעשות רצון הבורא ית' ולכן המה עדים נאמנים על הבורא בראותן עושים רצון קונם הכל מבינים כמה גדלה כבודו ית' שבנ"י נותנין נפשם על קדושת שמו

But Bnei Yisrael give over their very neshamos for the sanctification of His Name, and their intent is not to perform the mitzvos in order to receive a reward for their own benefit, but only to carry out the will of the Creator, Blessed is He; and therefore they are trustworthy witnesses concerning the Creator, for upon seeing them carrying out the will of their Maker, all comprehend how very great is His honor, Blessed is He, that Bnei Yisrael give over their neshamos for the sanctification of His Name.

Because Bnei Yisrael serve Hashem with mesiras nefesh, purely to fulfill the will of the Creator and not for any reward, they are trustworthy witnesses to His greatness, for all who behold their avodah grasp how exalted the honor of Hashem must be to inspire such devotion.

לכן אם הי' בעבור קבלת פרס לא הי' עדות כשר כיון שיש להם נגיעה בזה כמובן:

Therefore, were it for the sake of receiving a reward, it would not be valid testimony, since they would have a vested interest in the matter, as is understood.

Were Bnei Yisrael serving Hashem merely to collect a reward, they would have a negiah and their testimony would be invalid; it is precisely their selfless avodah, free of any personal benefit, that qualifies them as His witnesses.

Summary: The Sfas Emes explains that the Ribono shel Olam chose Bnei Yisrael to be His witnesses in the world, as the passuk says "You are My witnesses," and gave them the Shabbos, which is called "sahadusa," as the vehicle of that testimony. Building on the principle that a witness with a negiah is disqualified, he contrasts the nations of the world — whose occasional good deeds are ultimately self-serving — with Bnei Yisrael, who give over their neshamos for the sanctification of His Name and serve Hashem not for reward but solely to carry out the will of the Creator. It is precisely this selflessness and mesiras nefesh that makes Bnei Yisrael trustworthy witnesses, for when others behold their avodah, all come to grasp how great is the honor of Hashem Yisbarach. Were their avodah aimed at receiving a reward, they would have a vested interest and their testimony would be invalid; their disinterested devotion is the very foundation of their role as witnesses to the Creator.