Here, I. God intimates to Moses his purpose of coming down upon mount Sinai, in some visible appearance of his glory, in a thick cloud (Exo 19:9); for he said that he would dwell in the thick darkness (Ch2 6:1), and make this his pavilion (Psa 18:11), holding back the face of his throne when he set it upon mount Sinai, and spreading a cloud upon it, Job 26:9. This thick cloud was to prohibit curious enquiries into things secret, and to command an awful adoration of that which was revealed. God would come down in the sight of all the people (Exo 19:11); though they should see no manner of similitude, yet they should see so much as would convince them that God was among them of a truth. And so high was the top of mount Sinai that it is supposed that not only the camp of Israel, but even the countries about, might discern some extraordinary appearance of glory upon it, which would strike a terror upon them. It seems also to have been particularly intended to put an honour upon Moses: That they may hear when I speak with thee, and believe thee for ever, Exo 19:9. Thus the correspondence was to be first settled by a sensible appearance of the divine glory, which was afterwards to be carried on more silently by the ministry of Moses. In like manner, the Holy Ghost descended visibly upon Christ at his baptism, and all that were present heard God speak to him (Mat 3:17), that afterwards, without the repetition of such visible tokens, they might believe him. So likewise the Spirit descended in cloven tongues upon the apostles (Act 2:3), that they might be believed. Observe, When the people had declared themselves willing to obey the voice of God, then God promised they should hear his voice; for, if any man be resolved to do his will, he shall know it, Joh 7:17.
II. He orders Moses to make preparation for this great solemnity, giving him two days' time for it.
1. He must sanctify the people (Exo 19:10), as Job, before this, sent and sanctified his sons, Job 1:5. He must raise their expectation by giving them notice what God would do, and assist their preparation by directing them what they must do. "Sanctify them," that is, "Call them off from their worldly business, and call them to religious exercises, meditation and prayer, that they may receive the law from God's mouth with reverence and devotion. Let them be ready," Exo 19:11. Note, When we are to attend upon God in solemn ordinances it concerns us to sanctify ourselves, and to get ready beforehand. Wandering thoughts must be gathered in, impure affections abandoned, disquieting passions suppressed, nay, and all cares about secular business, for the present, dismissed and laid by, that our hearts may be engaged to approach unto God. Two things particularly prescribed as signs and instances of their preparation: - (1.) In token of their cleansing themselves from all sinful pollutions, that they might be holy to God, they must wash their clothes (Exo 19:10), and they did so (Exo 19:14); not that God regards our clothes; but while they were washing their clothes he would have them think of washing their souls by repentance from the sins they had contracted in Egypt and since their deliverance. It becomes us to appear in clean clothes when we wait upon great men; so clean hearts are required in our attendance on the great God, who sees them as plainly as men see our clothes. This is absolutely necessary to our acceptably worshipping God. See Psa 26:6; Isa 1:16-18; Heb 10:22. (2.) In token of their devoting themselves entirely to religious exercises, upon this occasion, they must abstain even from lawful enjoyments during these three days, and not come at their wives, Exo 19:15. See Co1 7:5.
2. He must set bounds about the mountain, Exo 19:12, Exo 19:13. Probably he drew a line, or ditch, round at the foot of the hill, which none were to pass upon pain of death. This was to intimate, (1.) That humble awful reverence which ought to possess the minds of all those that worship God. We are mean creatures before a great Creator, vile sinners before a holy righteous Judge; and therefore a godly fear and shame well become us, Heb 12:28; Psa 2:11. (2.) The distance at which worshippers were kept, under that dispensation, which we ought to take notice of, that we may the more value our privilege under the gospel, having boldness to enter into the holiest by the blood of Jesus, Heb 10:19.
3. He must order the people to attend upon the summons that should be given (Exo 19:13): "When the trumpet soundeth long then let them take their places at the foot of the mount, and so sit down at God's feet," as it is explained, Deu 33:3. Never was so great a congregation called together, and preached to, at once, as this was here. No one man's voice could have reached so many, but the voice of God did.

Sanctify them - See the meaning of this term, Exo 13:2.
Let them wash their clothes - And consequently bathe their bodies; for, according to the testimony of the Jews, these always went together.
It was necessary that, as they were about to appear in the presence of God, every thing should be clean and pure about them; that they might be admonished by this of the necessity of inward purity, of which the outward washing was the emblem.
From these institutions the heathens appear to have borrowed their precepts relative to washings and purifications previously to their offering sacrifice to their gods, examples of which abound in the Greek and Latin writers. They washed their hands and clothes, and bathed their bodies in pure water, before they performed any act of religious worship; and in a variety of cases, abstinence from all matrimonial connections was positively required, before a person was permitted to perform any religious rite, or assist at the performance.

And be ready against the third day,.... Not the third day of the month, but the third day from hence, this being the fourth, and the morrow the fifth, and the third day, the day following that, the sixth, on which day it is generally agreed by the Jews that the law was given; see Gill on Exo 19:16.

for the third day the Lord will come down in the sight of all the people upon Mount Sinai; which must be understood, consistent with his omnipresence, and is only expressive of some visible display of his power, and of some sensible token of his presence to the people; he was now upon it in the pillar of cloud, but then he would appear in another manner, and descend in a thick cloud and fire, which all the people would see, though they could not see the similitude of anything in it.

The 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.