In Exodus 19, Yahweh is about to reveal himself to the Israelites. He has told them that they have an opportunity to become his chosen people, a nation of priests for the rest of the world (vv. 5-6). The Lord gives them three days in which they are to consecrate themselves—no sex, wash their clothes, prepare their hearts (vv. 10-15). Then on the third day the Lord comes. There are very clear rules to protect the people from God’s presence—don’t touch the mountain or the foot of the mountain (v. 12).
On the morning of the third day there was thunder and lightning, with a thick cloud over the mountain, and a very loud trumpet blast. Everyone in the camp trembled. Then Moses led the people out of the camp to meet with God, and they stood at the foot of the mountain. Mount Sinai was covered with smoke, because the Lord descended on it in fire. The smoke billowed up from it like smoke from a furnace, and the whole mountain trembled violently. As the sound of the trumpet grew louder and louder, Moses spoke and the voice of God answered him. (Ex 19:16-19)
They tremble because they are in awe of God. They have seen what he can do to those who oppose him (like Pharoah). He’s not to be toyed with. We are not to be flippant in his presence. God begins his post-Exodus relationship with Israel by revealing himself as holy. And in Leviticus he says, “You are to be holy to me because I, the Lord, am holy, and I have set you apart from the nations to be my own” (Lev 20:26).
If you're like me, you have skimmed through Leviticus, or just skipped it all together. But that is because we don't understand what God is doing for the people of Israel.
[C]ompassion animates the Jewish purity system; it was a protective and benevolent system intended to preserve God’s presence among his people, a presence that could be of considerable danger to humans if they approached God wrongly… Access to sacred space was heavily restricted, not out of a lack of compassion but out of the belief that this holy God not only was merciful and loving but also was a powerful force that could be dangerous. 
God creates a system whereby the people of Israel could worship him and survive. There are categories of clean and unclean, holy and profane. The Sabbath is holy, Tuesday is profane (secular). A clean person who has sexual relations is temporarily unclean. Being unclean was not sinful, but a state in which you would not enter sacred space—like the Tabernacle or the Temple.
How serious was this? I’ll give you three short examples.
Aaron's sons offered unauthorized fire, “So fire came out from the presence of the Lord and consumed them!” (Lev. 10:2)
Uzzah sought to steady the ark when David was bringing it back to Jerusalem, “The Lord’s anger burned against Uzzah because of his irreverent act; therefore God struck him down.” (2 Sa 6:7)
And my favorite. Isaiah comes before God’s presence and is undone. Listen to David deSilva explain:
The story of Isaiah’s prophetic commissioning (Is 6:1-8) is a stunning example first of the awesomeness of the experience of the holy and then of the vibrant and overwhelming sense of danger and dread that overtakes the unclean one who enters the presence of the holy God: “Woe is me! I am lost, for I am a man of unclean lips, and I live among a people of unclean lips; yet my eyes have seen the King, the LORD of hosts” (Is 6:5). Isaiah has crossed too far into the presence of holiness, given his state of uncleanness, ... Rather than be consumed by God’s holiness, Isaiah is cleansed from his pollution by God’s seraph. 
In the first two examples, you can only imagine the fear of the Lord that came on the people. While God’s judgment may seem harsh to us, the son's of Aaron was glib and carefree with the holy space. Uzzah just didn't understand that the ark housed God's presence. They did not discern his holiness.
The Original Safe Space
God set up the Levitical priesthood so Israel could safely fellowship with him. But every so often, the people would forget and break the rules. Judgment would soon follow. But when the people honored the system, blessing would come. Far from being cold, dead and boring, we see images of the glory of God filling the Tabernacle and the Temple. Once David sufficiently discerned the holiness of God, he brought the ark to Jerusalem. While it killed Uzzah, the same ark brought blessing to the house of Obed-Edom (where they left the ark). If we honor God's presence, lesson will follow.
David was afraid of the Lord that day and said, “How can the ark of the Lord ever come to me?” He was not willing to take the ark of the Lord to be with him in the City of David. Instead, he took it to the house of Obed-Edom the Gittite. The ark of the Lord remained in the house of Obed-Edom the Gittite for three months, and the Lord blessed him and his entire household. (2 Sa 6:9–11)
Suddenly, David gets it. This time, he has more reverence.
But there was one significant difference between the two attempts to transport the sacred throne; this time Levites carried it by hand, not transporting it on a cart. Costly fellowship offerings consisting of “a bull and a fattened calf” were offered to the Lord after the Levites “had taken six steps.” This ritual pause after six steps suggests a symbolic significance, perhaps a sort of Sabbath rest, suggesting a consecration of the entire journey. 
Lie and Die
We see two opposite stories about holiness in the New Testament.
After Jesus’s miraculous fishing expedition, Simon Peter realizes he's with no ordinary rabbi and “he fell at Jesus’ knees and said, ‘Go away from me, Lord; I am a sinful man!’”  The story shows that Peter is a good Jew. He understands the consequences of being unclean in the presence of God. “This was an appropriate posture in a theophany,”  when God appears in human form.
However, we see the opposite with Ananias and Sapphira. They merely lied and suddenly died. And, “Great fear seized the whole church” (Acts 5:11).
You might be wondering how telling a lie evoked such judgment. How is it that any of us are alive? Well, this was a little different. I believe there is a principal at play here. The closer God is to a community, the greater the blessing. But also, there is a greater potential for judgment if we do not fear the Lord. In the height of the greatest revival in Israel’s history, when people were selling properties and giving the funds to the apostles to use as was needed, this couple lied about the price they received. They wanted to keep back some of the money for themselves. I’m sure many believers at that time felt it was harsh, but I also think that they received a greater understanding of the awesome nature of God.
This is not to make you afraid. Jesus calls as his friends and says, “Greater love has no one than this: to lay down one’s life for one’s friends” (John 15:13). We are to live with the understanding that our friend died for us. We have to live a life worthy of his death. Not flippant.
When we rightly discern his holiness, we can enjoy him the most. It is then, that he can draw close to us, and we can boldly approach him (Heb. 4:16), without fear (1 John 4:18). We can be like David, who danced his way to Jerusalem in the joy of the Lord.
 Thiessen, Matthew. Jesus and the Forces of Death (p. 11). Baker Publishing Group. Kindle Edition.
 deSilva, David A.. Honor, Patronage, Kinship, & Purity (p. 287). InterVarsity Press. Kindle Edition.
 Robert D. Bergen, 1, 2 Samuel, vol. 7, The New American Commentary (Nashville: Broadman & Holman Publishers, 1996), 331.
 The New International Version (Grand Rapids, MI: Zondervan, 2011), Lk 5:8.
 Robert H. Stein, Luke, vol. 24, The New American Commentary (Nashville: Broadman & Holman Publishers, 1992), 169.