With all preparations for the Tabernacle completed, Aaron and his sons were formally anointed to serve as priests. Aaron, and those who would succeed him as high priest (see the Fact Finder question below), all symbolized the coming ultimate Anointed One, Jesus Christ; in Greek, "Christ" means anointed one (see The Anointed One and What Is Jesus Christ Doing Right Now?).
"The Lord said to Moses [see also The Trysting Tent], "Take Aaron and his sons with him, and the garments, and the anointing oil, and the bull of the sin offering, and the two rams, and the basket of unleavened bread; and assemble all the congregation at the door of the tent of meeting." And Moses did as The Lord commanded him; and the congregation was assembled at the door of the tent of meeting.
Leviticus Chapter 9
"On the eighth day Moses called Aaron and his sons and the elders of Israel [see also The Senate]; and he said to Aaron, "Take a bull calf for a sin offering, and a ram for a burnt offering, both without blemish, and offer them before The Lord. And say to the people of Israel, 'Take a male goat for a sin offering, and a calf and a lamb, both a year old without blemish, for a burnt offering, and an ox and a ram for peace offerings, to sacrifice before The Lord, and a cereal offering mixed with oil; for today The Lord will appear to you.'" (Leviticus 9:1-4 RSV)
Leviticus Chapter 10
The Lord (see YHVH, Adonai, Jehovah, LORD) had provided the Levites with a precise code of conduct - and a warning about what would happen if it were violated. The priests were symbols of Christ, and as such could not take it upon themselves to do something on their own, thereby making themselves "false Christs" (see False Christs Of The Old Testament). Unfortunately for two of Aaron's sons, they disregarded their instructions and warnings.
"Now Nadab and Abihu, the sons of Aaron, each took his censer, and put fire in it, and laid incense on it, and offered unholy fire before The Lord, such as He had not commanded them. And fire came forth from the presence of The Lord and devoured them, and they died before The Lord. Then Moses said to Aaron, "This is what The Lord has said, 'I will show Myself Holy among those who are near Me, and before all the people I will be glorified.'" And Aaron held his peace." (Leviticus 10:1-3 RSV)