A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
exo 19:1ARRIVAL AT SINAI. (Exo. 19:1-25)
In the third month--according to Jewish usage, the first day of that month--"same day."--It is added, to mark the time more explicitly, that is, forty-five days after Egypt--one day spent on the mount (Exo 19:3), one returning the people's answer (Exo 19:7-8), three days of preparation, making the whole time fifty days from the first passover to the promulgation of the law. Hence the feast of pentecost, that is, the fiftieth day, was the inauguration of the Old Testament church, and the divine wisdom is apparent in the selection of the same reason for the institution of the New Testament church (Joh 1:17; Act 2:1).
exo 19:2were come to the desert of Sinai--The desert has its provinces, or divisions, distinguished by a variety of names; and the "desert of Sinai" is that wild and desolate region which occupies the very center of the peninsula, comprising the lofty range to which the mount of God belongs. It is a wilderness of shaggy rocks of porphyry and red granite, and of valleys for the most part bare of verdure.
and there Israel camped before the mount--Sinai, so called from Seneh, or acacia bush. It is now called Jebel Musa. Their way into the interior of the gigantic cluster was by Wady Feiran, which would lead the bulk of the hosts with their flocks and herds into the high valleys of Jebel Musa, with their abundant springs, especially into the great thoroughfare of the desert--the longest, widest, and most continuous of all the valleys, the Wady-es-Sheikh, while many would be scattered among the adjacent valleys; so that thus secluded from the world in a wild and sublime amphitheatre of rocks, they "camped before the mount." "In this valley--a long flat valley--about a quarter of a mile in breadth, winding northwards, Israel would find ample room for their encampment. Of all the wadys in that region, it seems the most suitable for a prolonged sojourn. The 'goodly tents' of Israel could spread themselves without limit" [BONAR].
exo 19:3Moses went up unto God--the Shekinah--within the cloud (Exo 33:20; Joh 1:18).
Thus shalt thou say to the house of Jacob, &c.--The object for which Moses went up was to receive and convey to the people the message contained in these verses, and the purport of which was a general announcement of the terms on which God was to take the Israelites into a close and peculiar relation to Himself. In thus negotiating between God and His people, the highest post of duty which any mortal man was ever called to occupy, Moses was still but a servant. The only Mediator is Jesus Christ [Ti1 2:5; Heb 12:24].
exo 19:6ye shall be unto me a kingdom of priests--As the priestly order was set apart from the common mass, so the Israelites, compared with other people, were to sustain the same near relation to God; a community of spiritual sovreigns.
an holy nation--set apart to preserve the knowledge and worship of God.
exo 19:7Moses came and called for the elders of the people--The message was conveyed to the mighty multitude through their elders, who, doubtless, instructed them in the conditions required. Their unanimous acceptance was conveyed through the same channel to Moses, and by him reported to the Lord. Ah! how much self-confidence did their language betray! How little did they know what spirit they were of!
exo 19:9The Lord said unto Moses, Lo, I come . . . in a thick cloud, &c.--The deepest impressions are made on the mind through the medium of the senses; and so He who knew what was in man signalized His descent at the inauguration of the ancient church, by all the sensible tokens of august majesty that were fitted to produce the conviction that He is the great and terrible God. The whole multitude must have anticipated the event with feelings of intense solemnity and awe. The extraordinary preparations enjoined, the ablutions and rigid abstinence they were required to observe, the barriers erected all round the base of the mount, and the stern penalties annexed to the breach of any of the conditions, all tended to create an earnest and solemn expectation which increased as the appointed day drew near.
exo 19:16on the third day in the morning, that there were thunders and lightnings, &c.--The descent of God was signalized by every object imagination can conceive connected with the ideas of grandeur and of awe. But all was in keeping with the character of the law about to be proclaimed. As the mountain burned with fire, God was exhibited as a consuming fire to the transgressors of His law. The thunder and lightning, more awful amid the deep stillness of the region and reverberating with terrific peals among the mountains, would rouse the universal attention; a thick cloud was an apt emblem of the dark and shadowy dispensation (compare Mat 17:5).
the voice of a trumpet--This gave the scene the character of a miraculous transaction, in which other elements than those of nature were at work, and some other than material trumpet was blown by other means than human breath.
exo 19:17Moses brought forth the people out of the camp to meet with God--Wady-er-Raheh, where they stood, has a spacious sandy plain; immediately in front of Es Suksafeh, considered by ROBINSON to be the mount from which the law was given. "We measured it, and estimate the whole plain at two geographical miles long, and ranging in breadth from one-third to two-thirds of a mile, or as equivalent to a surface of one square mile. This space is nearly doubled by the recess on the west, and by the broad and level area of Wady-es-Sheikh on the east, which issues at right angles to the plain, and is equally in view of the front and summit of the mount. The examination convinced us that here was space enough to satisfy all the requisitions of the Scripture narrative, so far as it relates to the assembling of the congregation to receive the law. Here, too, one can see the fitness of the injunction to set bounds around the mount, that neither man nor beast might approach too near, for it rises like a perpendicular wall." But Jebel Musa, the old traditional Sinai, and the highest peak, has also a spacious valley, Wady Sebaiyeh, capable of holding the people. It is not certain on which of these two they stood.
exo 19:21the 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.