Charles Ellicott Commentary Exodus 19:12

"And thou shalt set bounds unto the people round about, saying, Take heed to yourselves, that ye go not up into the mount, or touch the border of it: whosoever toucheth the mount shall be surely put to death:" — Exodus 19:12 (ASV)

You shall set bounds. —Here was another formal and mechanical direction, having for its object to deepen and intensify the lesson of God’s unapproachable majesty and holiness. Moses was required to set bounds to the people, i.e., to make a substantial fence between the camp and the base of Sinai, which should prevent both animals and men from coming in contact with the mountain.

Modern travellers generally observe how abruptly the rocky precipice of Ras Sufsafeh rises from the plain in front of it, so that in many places it is quite possible to stand on the plain and yet touch the mountain. The idea that a line of natural mounds now to be seen near the base of Sinai represents the “bounds” of Moses (Stanley, Sinai and Palestine, p. 43) is unsatisfactory, since the “bounds” of Moses were most certainly artificial ones. It is, however, possible that his “bounds” may have followed the line of the natural barrier.

That you go not up into the mount. —Unless it had been forbidden, there might have seemed to be no reason why pious Israelites might not have ascended the height, to draw near to God in prayer. It is a praiseworthy feeling which breathes in the words, “Nearer, my God, to thee;” but the nation was not fit for close approach.