Exodus 19

Exodus 19:1 in the third month… the same day came they into the wilderness of Sinai
Remember that the burning bush was at Sinai—that Moses had been living there with his sons, his wife, Zipporah, and her father (Jethro or Reuel) before he came to battle with Pharaoh.  In a way, Moses is bring the people back home, at least what had been home to him.
Russell M. Nelson
The journey from Egypt to Mount Sinai took about three months (see Ex. 19:1). The journey from Winter Quarters to the valley of the Great Salt Lake also took about three months (111 days). The destination for each group was described by the Lord as a land flowing with milk and honey.  The pioneers turned their wilderness into a fruitful field and made the desert blossom as a rose—precisely as prophesied by Isaiah centuries before (see Isa. 32:15–16; Isa. 35:1). (“The Exodus Repeated,” Ensign, July 1999, 9)
Exodus 19:2 there Israel camped before the mount
Mount Sinai had been the place where Moses met Jehovah.  His great calling as a deliverer like unto the Messiah came from the burning bush.  All prophets hope to bring their people into the presence of God.  This may be one of Moses’ motives; he had met God there, he had hopes to give the people a similarly miraculous experience with the divine—to bring them into the very presence of God.  While the people hadn’t been perfect up this point, they were guilty of no great sin.  There was some apostasy in Egypt to be sure, but there was no reason why the Lord’s greatest blessings couldn’t be given to the Israelites.  The time is a time of great hope for Moses.
Joseph Smith
Moses sought to bring the children of Israel into the presence of God, through the power of the Priesthood, but he could not. (Teachings of the Prophet Joseph Smith, 159)
Exodus 19:5 ye shall be a peculiar treasure unto me above all people
Russell M. Nelson
The phrase "peculiar treasure" was translated from the Hebrew segullah, meaning a highly valued possession--a "treasure." (Liahona, October 2022, 8)
Bruce R. McConkie
Because their numbers were known and the days of their mortal probation were selected in advance, Moses was able to say: "When the most High divided to the nations their inheritance, when he separated the sons of Adam, he set the bounds of the people according to the number of the children of Israel. For the Lord's portion is his people; Jacob is the lot of his inheritance." (Deut. 32:8-9.) And thus Jehovah said to Israel anciently: "If ye will obey my voice indeed, and keep my covenant, then ye shall be a peculiar treasure unto me above all people: for all the earth is mine: And ye shall be unto me a kingdom of priests, and an holy nation." (Ex. 19:5-6.) "For thou art an holy people unto the Lord thy God: the Lord thy God hath chosen thee to be a special people unto himself, above all people that are upon the face of the earth." (Deut. 7:6; 14:2.) And thus Peter said to Israel in his day: "Ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvelous light." (1 Pet. 2:9.) And as it was in those days, so it is today. Gathered Israel is now and everlastingly shall be a holy nation, a peculiar people, and a kingdom of priests who minister salvation to the peoples of the world.
Israel are the seed of Abraham; they are the children of the prophets; and they associate with the Lord's seers. Israel are the friends of apostles and revelators; they are the children of God by faith; they are the sons and daughters of the Lord Jesus Christ in whose name they worship the Father. Paul acclaims that they are the ones "to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises." They are the nation "of whom as concerning the flesh Christ came." (The Millennial Messiah: The Second Coming of the Son of Man [Salt Lake City: Deseret Book Co., 1982], 183)
Joseph Smith
When the children of Israel were chosen with Moses at their head, they were to be a peculiar people, among whom God should place His name; their motto was: "The Lord is our lawgiver; the Lord is our Judge; the Lord is our King, and He shall reign over us." (Isa. 33:22) While in this state they might truly say, "Happy is that people, whose God is the Lord." (Ps 144:15) Their government was a theocracy; they had God to make their laws, and men chosen by Him to administer them; He was their God, and they were His people. Moses received the word of the Lord from God Himself; he was the mouth of God to Aaron, and Aaron taught the people, in both civil and ecclesiastical affairs; they were both one, there was no distinction; so will it be when the purposes of God shall be accomplished: when "the Lord shall be King over the whole earth," and "Jerusalem His throne." "The law shall go forth from Zion, and the word of the Lord from Jerusalem."
This is the only thing that can bring about the "restitution of all things spoken of by all the holy Prophets since the world was"—"the dispensation of the fullness of times, when God shall gather together all things in one." (History of The Church of Jesus Christ of Latter-day Saints, 7 vols. 5:64)
Exodus 19:6 ye shall be unto me a kingdom of priests
The translation that comes through Peter is a “royal priesthood” (1 Pet. 2:9), but the Exodus term is “a kingdom of priests.”  The idea is similar, but there are significant differences.  Nowhere else in scripture do we find the phrase, “kingdom of priests.”  What does it mean?
It is crucial to realize that the Law of Moses had not yet been given.  The Levitical Priesthood had not yet been separated from the higher priesthood.  And the people were eligible for the greatest of God’s blessings under the covenant of Abraham.  We are used to thinking of the Israelites as recipients of the lesser law and the lesser priesthood and the lesser temple ordinances, but at this point in time as they camp before Sinai, the lesser status is not a foregone conclusion.  Anything is still possible.
Now consider the phrase, “kingdom of priests.”  God is not asking for a tribe of priests (i.e. Levites); He wants an entire kingdom of priests (see also Num. 11:29).  These would be priests under the Melchizedek priesthood (If you think there is no such thing as a Melchizedek Priesthood priest, then how do you explain Heb. 5:6? Also, remember that the peoples of the Book of Mormon had only the Melchizedek Priesthood since Lehi was not a Levite. Their priest and high priest callings were of that order and not of the Aaronic.).  This is a remarkable clue that the Lord intends to give the priesthood to all the tribes, and for the time being, it is the Melchizedek Priesthood.  It hints at what we could only know through the Joseph Smith Translation—that the Lord first intended to give them the higher law and the higher priesthood—that is until the golden calf incident (JST Ex. 34:1-2).
Does God want the same blessing for us?  Doesn’t God invite all the faithful to be foreordained as “kings and priests unto the Most High God”?  Or do you prefer the term, “queens and priestesses”?  What is the purpose of an entire church of kings, priests, queens, and priestesses?  Isn’t it to rule and reign in the house of Israel forever?  God was trying to get these Israelites to the same high and holy calling, but they blew it.  Plain and simple, they had their chance and messed it up!
Hopefully, we won’t make the same mistake!
Russell M. Nelson
Upon our shoulders lies the responsibility to keep the faith through our own generation. This “ball” is now in our court! We of modern Israel are destined to be “a kingdom of priests, and an holy nation” (Ex. 19:6). (“The Exodus Repeated,” Ensign, July 1999, 13)
Joseph Smith
The God of heaven has begun to restore the ancient order of his kingdom unto his servants and his people—a day in which all things are concurring to bring about the completion of the fulness of the Gospel, a fulness of the dispensation of dispensations, even the fulness of times; a day in which God has begun to make manifest and set in order in his Church those things which have been, and those things which the ancient prophets and wise men desired to see but died without beholding them; a day in which those things begin to be made manifest, which have been hid from before the foundation of the world, and which Jehovah has promised should be made known in his own due time unto his servants, to prepare the earth for the return of his glory, even a celestial glory, and a kingdom of priests and kings to God and the Lamb, forever, on Mount Zion, and with him the hundred and forty and four thousand whom John the Revelator saw, all of which is to come to pass in the restitution of all things. (italics added, History of the Church, 4:492-93.)
Joseph Smith
The Saints should be a select people, separate from all the evils of the world—choice, virtuous, and holy. The Lord was going to make of the Church of Jesus Christ a kingdom of Priests, a holy people, a chosen generation, as in Enoch's day, having all the gifts as illustrated to the Church in Paul's epistles and teachings to the churches in his day—that it is the privilege of each member. (History of The Church of Jesus Christ of Latter-day Saints, 7 Vols. 4:570)
Exodus 19:7-8  the elders of the people… said, All that the Lord hath spoken we will do
This is before Moses has gone up the Mount so we are not sure exactly what the command is referenced in verse 7, but the elders make a promise to God, a covenant as it were, to do “All that the Lord hath spoken.”  This is the Lord placing the Israelites under covenant. 
Now if you are bothered by the fact that it is the elders covenanting on behalf of the people instead of the people doing it themselves, we read later that “Moses came and told the people all the words of the Lord and all the judgments: and all the people answered with one voice, and said, All the words which the Lord hath said will we do.” (Ex. 24:3, italics added)
“Exodus gives a priceless close-up of how important covenant making is to salvation. Moses received the Ten Commandments and inspired explanations, wrote this body of revealed law in a sacred record, and then read it to the people, who entered into a ceremonial commitment:
 “He took the book of the covenant, and read in the audience of the people: and they said, All that the Lord hath said will we do, and be obedient.
 “And Moses took the blood, and sprinkled it on the people, and said, Behold the blood of the covenant, which the Lord hath made with you concerning all these words.” (Ex. 24:7–8.)
(Richard Lloyd Anderson, Ensign, Feb. 1992, 14)
Exodus 19:11 be ready against the third day: for the third day the Lord will come down in the sight of all the people
When God comes to commune with his people, he asks for some preparation and sanctification.  He visited the Nephites but asked them to prepare for day two saying, “go ye unto your homes, and ponder upon the things which I have said, and ask of the Father, in my name that ye may understand, and prepare your minds for the morrow, and I come unto you again.” (3 Ne 17:3)
Joshua said to the people: "Sanctify yourselves: for to morrow the Lord will do wonders among you." (Josh. 3:5.)
sinai smoke_0.png
Exodus 19:18 the mount Sinai was altogether on a smoke, because the Lord descended upon it in fire
Moses later recounts how dramatic a scene this was.  He has to remind the people what they had seen and heard on that momentous occasion. 
And ye came near and stood under the mountain; and the mountain burned with fire unto the midst of heaven, with darkness, clouds, and thick darkness.
And the LORD spake unto you out of the midst of the fire: ye heard the voice of the words, but saw no similitude; only ye heard a voice.
And he declared unto you his covenant, which he commanded you to perform, even ten commandments; and he wrote them upon two tables of stone.
The LORD talked with you face to face in the mount out of the midst of the fire,
(I stood between the LORD and you at that time, to shew you the work of the LORD: for ye were afraid by reason of the fire, and went not up into the mount;) saying,
And ye said, Behold, the LORD our God hath shewed us his glory and his greatness, and we have heard his voice out of the midst of the fire: we have seen this day that God doth talk with man, and he liveth.
Now therefore why should we die? for this great fire will consume us: if we hear the voice of the LORD our God any more, then we shall die.
For who is there of all flesh, that hath heard the voice of the living God speaking out of the midst of the fire, as we have, and lived?
Go thou near, and hear all that the LORD our God shall say: and speak thou unto us all that the LORD our God shall speak unto thee; and we will hear it, and do it.
And the LORD heard the voice of your words, when ye spake unto me; and the LORD said unto me, I have heard the voice of the words of this people, which they have spoken unto thee: they have well said all that they have spoken.
O that there were such an heart in them, that they would fear me, and keep all my commandments always, that it might be well with them, and with their children for ever! (Deut. 4:11-13; 5:4-5, 24-29)
Neal A. Maxwell
I testify that the Jehovah introduced by thunderings and lightnings to a gathered Israel at Sinai (see Ex. 19:16–18) is the same Jesus who later lamented, “O Jerusalem, Jerusalem … how often would I have gathered thy children together, even as a hen gathereth her chickens under her wings.” (Matt. 23:37.) I thank him for such repeated reachings out to mankind, whether in phenomenal power or in quiet conversation at a wellside. (Ensign, May 1976, 27)