Thou shalt set bounds. The erection of a fence or barrier, between the camp and the mountain—not necessarily all round the mountain—seems to be meant. This barrier may have run along the line of low alluvial mounds at the foot of the cliff of Ras Sufsafeh, mentioned by Dean Stanley, but cannot have been identical with them, since it was an artificial fence.
That ye go not up into the mount. Curiosity might have tempted some to ascend the mount, if it had not been positively forbidden under the penalty of death; carelessness might have brought many into contact with it, since the cliff rises abruptly from the plain.
Unless the fence had been made, cattle would, naturally, have grazed along its base. To impress the Israelites with a due sense of the awful majesty of God, and the sacredness of everything material that it brought into close relations with him, the mount itself was declared holy—none but Moses and Aaron might go up into it; none might touch it; even the stray beast that approached it must suffer death for its unwitting offence (Exodus 19:13).
Whosoever toucheth the mount. The mountain may be "touched" from the plain—it rises so abruptly. Shall be surely put to death. A terrible punishment, and one which, to modern ideas, seems excessive. But it was only by terrible threats, and in some cases by terrible punishments (2 Samuel 6:7), that the Israelites could be taught reverence.
A profound reverence lies at the root of all true religious feeling; and for the education of the world, it was requisite, in the early ages, to inculcate the necessity of this frame of mind in some very marked and striking way.