Most of us know the story of Nadab and Abihu: "Now Nadab and Abihu, the sons of Aaron, each took his censer and put fire in it and laid incense on it and offered unauthorized fire before the LORD, which he had not commanded them. And fire came out from before the LORD and consumed them, and they died before the LORD." (Lev. 10:1-2) This occurred on the day that Aaron and his sons completed the week long process which consecrated them as priests. What a start! It was a sharp lesson that God expected more of those who have the closest relationship to Him. "Then Moses said to Aaron, “This is what the LORD has said: ‘Among those who are near me I will be sanctified, and before all the people I will be glorified.’” And Aaron held his peace." (Lev. 10:3)
When God gave Moses the instructions for building the tabernacle and all the furniture in it, He emphasized the importance of the altar of incense. Alone of all the things in the tabernacle, the altar of incense is said to be "most holy unto the LORD," and the High Priest must "make atonement upon the horns of it once in the year" (Ex. 30:10). A careful warning is also made: "You shall not offer unauthorized incense on it, or a burnt offering, or a grain offering, and you shall not pour a drink offering on it." (Ex. 30:9) Add to that warning a command that the fire on the altar of burnt offering should never go out (Lev. 6:12-13) and the event witnessed by all that God Himself lighted the altar of burnt offering (Lev. 9:24), and the conclusion should have been reached that they should be careful with these things. So when Nadab and Abihu offer incense with unauthorized fire, they pay the price for not treating God with the fear and reverence He deserves.
Immediately after the explanation and instructions regarding Nadab and Abihu's deaths God says that priests should never enter the tabernacle having drunk wine "that you die not" (vs 9). Given the events that had JUST taken place, Aaron knew that the threat of death wasn't just an absent-minded emphasis. God wasn't just speaking strongly to make a point. If they did not sanctify and glorify Him in their actions, death awaited.
Leviticus also teaches the lesson to take care in following the instructions of God. Here God gives the delineation between clean and unclean animals. "Whatever parts the hoof and is cloven-footed and chews the cud, among the animals, you may eat." (vs 3) That instruction should have been enough, but God painstakingly emphasizes the rule. The camel, rock badger and hare all chew the cud but don't part the hoof and are therefore unclean. The swine parts the hoof, but does not chew the cud and is unclean. In other words, a partial obedience to God's instructions doesn't cut it. For God, there is no such thing as "close enough".
Since modern Christians are called a "royal priesthood" by Peter (2:9), we bear the same burden as Aaron and his sons. Who are closer to God than the members of His family? (Rom. 8:16, Eph. 2:16) Since the law was to bring us to Christ (Gal. 3:24) and is profitable for the perfection of the man of God (2 Tim. 3:16), then the principles it teaches about how to approach God are viable. Therefore, we must show by our lives that God is holy. Our lives should bring glory to God in the sight of the world. Part of so honoring God is to take care to follow His instructions exactly--not to be saved by our rule keeping, but to demonstrate that God's wishes are important. That they are THAT important. All of which explains why we are so careful about how and when we worship God and why we are so strict about how the church's money is used. It is why we should be careful in our daily lives: about how we speak, what we laugh at, how our free time is used and how we raise our children. To sanctify and glorify God (Lev. 10:3) ought to be reason enough for a people saved by the sacrifice of God's Son, but perhaps an even baser reason will resonate: "that [we] die not".
2 Thess. 1:7-8 "when the Lord Jesus is revealed from heaven with his mighty angels in flaming fire, inflicting vengeance on those who do not know God and on those who do not obey the gospel of our Lord Jesus."