Exodus 19:12

Set bounds. Moses was to erect a barrier of some kind about the base of the mountain. It may have extended along the line of low mounds guarding the foot of the cliff. So long as the people refrained from crossing these “bounds” they were safe.

It has been fittingly remarked that Sinai, with all its described terrors, was not Vesuvius; the people beneath were not gathered in a doomed Herculaneum or Pompeii. The purpose of the Lord was simply to manifest the reality, extent, and proximity of His destroying power. Men were made to feel what that power could do, if they were so presumptuous or negligent as to come within its rightful exercise.

Whosoever toucheth. Because the cliff rises abruptly from the plain, some might easily come in contact with it, either carelessly or out of curiosity.

Be surely put to death. This severe punishment was specifically announced by God to impress upon the people in no uncertain terms what it meant to be in the presence of a holy God. The penalty was fitting in keeping with the awful solemnity of the occasion. Further, we should not forget that it was only by sobering threats of punishment that the Israelites, prone at times to be presumptuous and to rebel against the divine will, could be taught reverence (see 2 Sam. 6:6, 7). There can be no true religious feeling without a profound sense of reverence. To inculcate such an attitude among the Israelites, it was imperative to impress the lesson in a dramatic and striking fashion.