When Moshe argues to G-d that Bnei Yisrael will not believe that he was sent as their redeemer, it sounds like a question local to the context of the story of yetzi’as Mitzrayim. Rambam (Yisodei HaTorah 8:2) however reads the whole conversation as a philosophical debate:
וזהו שאמר לו הקב"ה בתחילת נבואתו בעת שנתן לו האותות לעשותן במצרים ואמר לו ושמעו לקולך. ידע משה רבינו שהמאמין על פי האותות יש בלבבו דופי ומהרהר ומחשב והיה נשמט מלילך ואמר והן לא יאמינו לי. עד שהודיעו הקב"ה שאלו האותות אינן אלא עד שיצאו ממצרים ואחר שיצאו ויעמדו על ההר הזה יסתלק הרהור שמהרהרין אחריך שאני נותן לך כאן אות שידעו שאני שלחתיך באמת מתחילה ולא ישאר בלבם הרהור
The Rambam in the first halacha of that chapter writes that signs and wonders, no matter how remarkable, do not prove the legitimacy of a prophet. The reason we believe in the prophecy of Moshe Rabeinu is because we witnessed ma’amad Har Sinai with our own eyes and heard G-d tell Moshe to speak to us.
At the burning bush, when G-d gave Moshe signs to prove that he was the real deal, Moshe countered with this philosophical point that signs are not sufficient proof of anything. Hashem answered that this was only a temporary measure. The real proof would come later at Sinai.
The Lechem Mishneh points out that the Rambam’s version of the conversation between Moshe and G-d actually inverts the order of the pesukim as they appear in chumash. The way the pesukim appear is as follows:
- G-d tells Moshe that there will be a ma’amad Har Sinai (3:12)
- Moshe argues that Bnei Yisrael will not believe him (4:1)
- Hashem gives Moshe three signs
According to Rambam, it’s exactly the reverse: first Hashem gave Moshe the signs, then Moshe questioned whether they are sufficient proof, and finally Hashem revealed that the ultimate true sign is ma’amad Har Sinai.
Be that as it may, what I find strange is that b’shalama according to the Maharal (Gevuros Hashem) who holds that ma’amad Har Sinai is just one ingredient in the cholent of what brings us to emunah, but we also need the miracle of yetzi’as Mitzrayim and kriyas Yam Suf to establish the notion of hashgacha, I understand why so many mitzvos are “zecher l’yetzi’as Mitzrayim.” The miracle is a proof. But according to the Rambam that everything is rooted only in ma’amad har Sinai and miracles and wonders prove nothing, even miracles as great as yetzi'as Mitzrayim, why are there no mitzvos “zecher l’ma’amad Har Sinai?” It’s not even counted as a mitzvah! Why is yetzi’as Mitzrayim and not ma’amad har Sinai the focus?
The Rambam ends that halacha by sneaking in a remarkable chiddush in hilchos eidus:
נמצאת אומר שכל נביא שיעמוד אחר משה רבינו אין אנו מאמינים בו מפני האות לבדו כדי שנאמר אם יעשה אות נשמע לו לכל מה שיאמר. אלא מפני המצוה שצוה משה בתורה ואמר אם נתן אות אליו תשמעון. כמו שצונו לחתוך הדבר על פי שנים עדים ואע"פ שאין אנו יודעין אם העידו אמת אם שקר. כך מצוה לשמוע מזה הנביא אע"פ שאין אנו יודעים אם האות אמת או בכישוף ולט:
The Rambam tells us that we can never know in reality who is a true prophet and who isn’t. Even if the prophet performs miracles, that's not proof. The only reason we rely on a prophet is because the Torah commands us to do so. In that same way, says the Rambam, we really never know if what two witnesses say is true or not. The only reason we rely on their testimony is because the Torah tells us to.
In other words: eidus is a hanhaga, not a birur.
I don't have the time or ability now to sink into this swamp of a topic, but here's one issue that exploits this chiddush: R’ Ya’akov Moshe Charlap (Beis Zevul I: 5: 7) quotes a teshuvah from the Nesivos who asks as follows: if a treif piece of meat fell into the cholent, you are allowed to eat the meat based on bitul b’rov and are chayav nothing if afterwards you figure out that you ate the treif piece. However, the Mishna in Yevamos (91) says that if a woman remarries based on the testimony of eidim (not an eid echad) who say her husband died and then her husband re-appears, she is chayeves a korban. Why the difference?
In a nutshell, the difference is that rov is a birur while eidus is only a hanhaga. As long as there is a rov of heter, the cholent becomes heter -– that is the reality. However, when it comes to eidim, we never know the truth. The Torah accepts testimony in lieu of factual knowledge. However, where that testimony proves false, the individual affected is liable for not doing more to ascertain the truth. It's still shogeg and not meizid, but it's not ones because the person knew full well that the truth remained unknown.