"Watch out for the LORD on the mountain!" said the LORD on the mountain. "When the LORD comes to this mountain," continued the LORD on the mountain, "you won't even be able to touch the mountain." (v10ff)
Of course these kinds of statements would be absurd if we assumed a uni-personal God in the OT. But they make perfect sense when we take seriously our theme verse from Exodus 3:12. It is the Divine Angel - the great I AM - who brings His people to the mountain to meet with God (Ex 3:12). The LORD Jesus saves a people to serve the Father.
Verses4-5: He has now brought them on eagles wings (cf Deut 32:11; 2 Sam 1:23; Ps 103:5; Is 40:31) as His treasured possession (Deut 7:6; 14:2; 26:18; Ps 135:4; Mal 3:17). Their election is for the sake of the world. They are the treasure in the field (cf Matt 13:44). The whole field is purchased by the LORD that He might set His special affection on the treasure. But that special-ness is a priestly special-ness. The world is purchased for the treasure and the treasure exists for the world.
The whole earth is the LORD's and Israel is His priest to the nations. The whole purpose for their existence is to bring the nations to God and God to the nations.
In the rest of chapter 19 we get a little picture of priestliness.
The whole nation is commanded to go up the mountain in v13. Yet when the trumpet blasts and then gets louder and louder (v19), the Israelites remain at the foot of the mountain. It seems to me that what the LORD (Jesus) says about the LORD (the Father) in v21 is a response to their reticence:
The LORD said to him, "Go down and warn the people so they do not force their way through to see the LORD and many of them perish. 22 Even the priests, who approach the LORD, must consecrate themselves, or the LORD will break out against them."
They were too scared to come up and then the LORD confirms their decision (that's how it seems to me). And so the nation remains at the foot of the mountain. The priests come a certain way up. And (v24) Moses and Aaron (and later Joshua) can come all the way up. Here is a kind of tabernacle division before the tabernacle.
The nations are right outside the camp. The Israelites can come a certain distance. The priests can come up further (with consecration). But the High Priest / Head / Joshua(Jesus) will go into the heart of the firey/cloudy Presence on behalf of the people.
From now on this kind of priestly access to God will be enshrined in the tabernacle and Levitical laws. This is what priesthood looks like. One body acting on behalf of the greater mass. And one man in particular summing up that priestly body.
In Deuteronomy 18 we see what the Israelites were to learn about this:
15 The LORD your God will raise up for you a prophet like me from among your own brothers. You must listen to him. 16 For this is what you asked of the LORD your God at Horeb on the day of the assembly when you said, "Let us not hear the voice of the LORD our God nor see this great fire any more, or we will die." 17 The LORD said to me: "What they say is good. 18 I will raise up for them a prophet like you from among their brothers; I will put my words in his mouth, and he will tell them everything I command him. 19 If anyone does not listen to my words that the prophet speaks in my name, I myself will call him to account.
The Mediating LORD who brought them to the mountain would one day be the Priestly LORD-Man. No more divisions within Israel between the Son, the High Priests, the priests and the people. He will be the people, the priest, the high priest, Moses, the temple, the sacrifice all rolled up in one! And we still remain the treasured priestly people (1 Peter 2:4-6). We have been brought all the way up the mountain by our ascended Priest. And now we exist as the priestly nation bringing the world to the unseen LORD through the Saving God-Man.
Sorry this was late. And rushed...