Rabbi Nathan presents this teaching from the Mekhilta DeRabbi Yishmael as a direct rebuttal to heretics who claim there are two divine powers. The argument is elegant in its simplicity.
When the Holy One Blessed be He stood at Mount Sinai and declared "I am the Lord your God," who stood up to contest Him? If there were truly a second deity, that would have been the moment to object. The entire nation of Israel was assembled. The heavens were open. God was making His singular claim to sovereignty. If any rival power existed, Sinai was the time and place to challenge it.
No one did. No rival voice spoke. No competing power made itself known.
But perhaps, the Mekhilta anticipates, someone might argue that the revelation happened in secret, in some hidden corner where a second deity would not have noticed. Rabbi Nathan dismisses this immediately by citing (Isaiah 45:19): "Not in secret did I speak." The revelation at Sinai was the most public event in history. God did not whisper the Torah to a select few. He thundered it before an entire nation, in the open, for all creation to witness.
The verse continues: "I, the Lord, speak righteously; I tell what is true." God's declaration of unity was not a private claim made behind closed doors. It was a public, verifiable, witnessed event. The absence of any rival voice at Sinai is itself the proof that no rival exists. There is one God, and He spoke openly, and no one contradicted Him.