Sforno on Exodus 19:9/ספורנו על שמות י״ט:ט׳

1בעב הענן, even though all of Moses’ prophetic stature underwent a drastic change on the occasion of the revelation at Mount Sinai, and he now received “clear” messages, something described in Numbers 12,8 as ותמונת ה' יביט, “he sees the message from G’d as a clear picture” (not merely as a reflection by a mirror) he was not to behold the Shechinah, but it was shrouded in thick cloud. The present communication with G’d which Moses used was not of the אספקלריא מאירה, “clear view” type.

2בעבור ישמע העם וגם בך יאמינו לעולם, they will believe in the possibility of this “face to face” communication between man and G’d, and the fact that you have been granted this level of prophecy by Me. (compare Deut.5,4 concerning the expression “face to face.”) One of the principal ingredients of this level of prophecy is that the prophet remains in full control of all his faculties including all parts of his body, something no one had been able to credit until they saw that Moses possessed such a capability.The Israelites commented on this in Deuteronomy 5,21 when Moses quoted them as having said: “this day we have seen that G’d can speak directly to man and man can survive such an encounter.” [In other words, it took the revelation at Mount Sinai, experienced by all the people, to teach them that Moses could enjoy such a relationship with G’d on an ongoing basis. Ed.] It is not that the people needed convincing that there were people who enjoyed prophetic stature and communicated with G’d and He with them. After all, they knew about all the patriarchs, Aaron and Miriam as well as Moses being prophets. However, up until this time all prophetic messages from G’d had been communicated either in a dream or in a vision involving the prophet losing touch with his surroundings at the time of such communication by G’d. [at the very least, if such a communication had been received while the recipient was awake he would have to sit down, as he would be unable to stand. [Avraham’s being able to run to welcome the angels, (his vision) was a sign of a higher than usual level of prophecy. Ed.] Now, for the first time, did the people witness that the recipient of such communications need not necessarily have to temporarily forego being in touch with all that goes on in our terrestrial world. You will understand this better in light of how G’d Himself explained to Miriam and Aaron in Numbers 12,7 the difference between their level of communication with G’d and that of their brother Moses, whom G’d described as thoroughly “at home” in all sections of “His house.” Only after having themselves experienced what this kind of prophetic level meant could they credit Moses as possessing it. Now they also understood that no other prophet, even a legitimate one, could challenge Moses, seeing that his stature as prophet was so superior to any other prophet.

3'ויגד משה את דברי העם אל ה, when he heard that his Creator had told him that G’d wanted the people to have absolute faith in the supremacy of his prophecy, Moses understood suddenly that the people had entertained some doubts about the nature of his prophetical powers. He conveyed the people’s words to G’d to show that they had unreservedly undertaken to accept everything G’d wanted from them as conveyed to them from him as G’d’s word. He emphasized that the people had not only said: כן נעשה, “so we shall do,” but had said כל אשר דבר ה' נעשה, “everything that G’d has said we shall do.” This was in fact a restrictive comment, meaning that if what they heard was G’d’s command they would do it, not if it was only the command of an angel. We know already from Exodus 23,21 that an angel is not empowered to forgive any errors committed by the people, כי לא ישא לפשעכם, and therefore Moses himself rejected the idea that the people be under the protection of an angel instead of under G’d’s personal supervision. The people were aware of this distinction. This is why they had committed themselves to obey only commands issued by G’d directly.