Jamieson, Fausset and Brown on Exodus 19:21

21. the Lord said unto Moses, Go down, charge the people--No sooner had Moses proceeded a little up the mount, than he was suddenly ordered to return, in order to keep the people from breaking through to gaze--a course adopted to heighten the impressive solemnity of the scene. The strict injunctions renewed to all, whatever their condition, at a time and in circumstances when the whole multitude of Israel were standing at the base of the mount, was calculated in the highest degree to solemnize and awe every heart.