Tafsiran Keluaran 19:12: Rashi, Chizkuni, Sforno & Or HaChaim

והגבלת AND THOU SHALT SET BOUNDS — set boundary marks for them as a sign that they should not approach the mountain from the boundary and beyond.

והגבלת את ההר, “you are to “fence in“ the mountain; actually the Torah writes: “you shall set boundaries for the people,” את העם. Our author understands this as fencing off the east side of the camp from the mountain. This is the direction from which the glory of G-d would appear, as we know from what Moses described in Deuteronomy 33,2 when he spoke about G-d having first offered His Torah to the gentile nations, who, upon enquiring what it contained decided to decline acceptance. He said then: ,וזרח משעיר למו הופיע מהר פרן, “He shone forth from the land of Seir (in the east) and He appeared from Mount Paran.” [But the Torah wrote סביב – fence the mountain roundabout; Cf. Mekhilta ad loc.]

כל הנוגע בהר מות ימות, a warning to deter the people from being harmed in the event they would trample the boundary markers around the mountain in their eagerness to catch a glimpse of G’d. Such people, if they were to become victims of their own folly, would mar the entire joyful experience of the giving of the Torah by defiling the earth around them with their carcasses. They would cause their relatives to become mourners on what should have been the most auspicious experience of their lives. The presence of G’d, the Shechinah, does not manifest itself to people in mourning.

לאמר SAYING — The boundary says to them, “Take care not to ascend from here and further on” and you, too, admonish them regarding this.

ונגע בקצהו OR TOUCH THE EXTREMITY THEREOF — i. e. even the extremity thereof.