God appointed Aaron and his sons to serve as His priests under the Law of Moses.
As GOD continued revealing His will to Moses upon Mount Sinai, He appointed Aaron and Aaron's sons to serve as priests.
1 “Now take Aaron your brother, and his sons with him, from among the children of Israel, that Aaron, Nadab, Abihu, Eleazar, and Ithamar may minister to Me.
(Exodus 28:1 OSB)
Once the tabernacle and its furnishings were made ready, Aaron and his four sons began their new roles as priests over the house of Israel.
Leviticus 8-9 recounts how Aaron and his sons began their mission as priests serving at the tabernacle. After dressing in the priestly clothes, Moses led them in ceremonial cleansing and then they spent a full week in the tabernacle making ready.
1 Now the Lord spoke to Moses, saying, 2 “Take Aaron and his sons with him, and the garments, the anointing oil, a young bull as the sin offering, two rams, and a basket of unleavened bread; 3 and gather all the congregation together at the door of the tabernacle of testimony.”
4 So Moses did as the Lord commanded him, and the congregation was gathered together at the door of the tabernacle of testimony. 5 Then Moses said to the congregation, “This is the word the Lord commanded us to do.” 6 So Moses brought Aaron and his sons and washed them with water. 7 He put the tunic on him, girded him with the belt, clothed him with the robe, and put the ephod on him; and he girded him with the intricately woven band of the ephod, and with it tied the ephod on him. 8 Then he put the breastplate on him, and he put the Urim and the Thummim in the breastplate. 9 He also put the turban on his head; and on the turban, on its front, he put the golden plate, the holy crown, as the Lord commanded Moses.
10 Moses then took the anointing oil, 11 and he sprinkled some of it on the altar seven times, anointed the altar, and sanctified it, and all its utensils, and the laver and its base, to sanctify them. He also anointed the tabernacle and everything in it, and sanctified it. 12 Moses then poured some of the anointing oil on Aaron's head and anointed him, to sanctify him. 13 After this, Moses brought Aaron's sons and put tunics on them, girded them with the belts, and put hats on them, as the Lord commanded Moses.
14 Then he brought the young bull for the sin offering; and Aaron and his sons put their hands on the head of the bull for the sin offering, 15 and Moses killed it. Then he took the blood, and put some on the horns of the altar all around with his finger, and purified the altar. He poured the blood at the base of the altar, and sanctified it, to make atonement upon it. 16 Moses then took all the fat on the entrails, the fatty lobe attached to the liver, and the two kidneys with their fat, and Moses offered them on the altar. 17 But the young bull, its hide, its flesh, and its offal he burned with fire outside the camp, as the Lord commanded Moses.
18 Next, Moses brought the ram as the burnt offering. Aaron and his sons put their hands on the head of the ram, 19 and Moses killed the ram. Then he poured the blood all around on the altar 20 and cut the ram into pieces; and he offered the head, the pieces, and the fat. 21 Then he washed the stomach and the feet in water and offered the whole ram on the altar. It was a whole burnt offering for a sweet aroma, a burnt offering to the Lord, as the Lord commanded Moses.
22 After this, Moses brought the second ram, the ram of consecration. Then Aaron and his sons put their hands on the head of the ram, 23 and Moses killed it. Also he took some of its blood and put it on the tip of Aaron's right ear, on the thumb of his right hand, and on the big toe of his right foot. 24 Then Moses brought Aaron's sons; and he put some of the blood on the tips of their right ears, on the thumbs of their right hands, and on the big toes of their right feet. Then he poured the blood all around on the altar. 25 He took the fat and the fat tail, all the fat on the entrails, the fatty lobe attached to the liver, the two kidneys and their fat, and the right thigh; 26 and from the basket of consecration before the Lord, he took one unleavened cake, a cake of bread anointed with oil, and one wafer and put them on the fat and on the right thigh; 27 and he put all these in Aaron's hands and in his sons’ hands and offered them as a choice portion before the Lord. 28 Then Moses took them from their hands and offered them on the altar for a whole burnt offering of consecration, which was a sweet aroma and a burnt offering to the Lord. 29 Moses also took the breast and removed it as a deposit from the ram of consecration before the Lord. It was Moses’ part, as the Lord commanded Moses.
30 Moses then took some of the anointing oil and some of the blood on the altar and sprinkled it on Aaron and his garments, and on his sons and their garments with him; and he sanctified Aaron and his garments, and his sons and their garments with him.
31 After this, Moses said to Aaron and his sons, “Boil the flesh in the court of the tabernacle of testimony, in a holy place, and eat it there with the bread in the basket of consecration as I was commanded, saying, ‘Aaron and his sons shall eat it.’ 32 What remains of the flesh and the bread shall be burned with fire. 33 You shall not go outside the door of the tabernacle of testimony for seven days, until the days of your consecration are fulfilled. For seven days he shall consecrate you. 34 As the Lord did on this day, He commanded to do to make atonement for you. 35 Therefore you shall stay at the door of the tabernacle of testimony day and night for seven days and keep the charge of the Lord, that you may not die; for the Lord God so commanded me.” 36 So Aaron and his sons did all the words the Lord commanded Moses.
(Leviticus 8:1-36 OSB)
As the week-long period of consecration ended, Aaron and his sons exited the tabernacle. GOD then made known to all the people of Israel His acceptance of new priests' consecration period, which, sadly was followed by tragedy due to rebellious disobedience by two of Aaron's sons.
1 Now it came to pass on the eighth day that Moses called Aaron and his sons and Israel's council of elders. 2 Moses said to Aaron, “Take for yourself a young calf from the oxen as a sin offering and a ram as a whole burnt offering, without blemish, and offer them before the Lord. 3 And to the council of elders you shall speak, saying, ‘Take one kid of the goats as a sin offering, and a young calf and a lamb, both of the first year, without blemish, as a whole burnt offering, 4 also a young bull and a ram as peace offerings before the Lord, and a grain offering mixed with oil; for today the Lord will appear to you.’”
5 So they brought what Moses commanded before the tabernacle of testimony, and all the congregation drew near and stood before the Lord. 6 Then Moses spoke this word, “This is the thing the Lord commanded you to do, and the glory of the Lord will appear among you.” 7 Moses then said to Aaron, “Go to the altar, offer your sin offering and your whole burnt offering, and make atonement for yourself and for your house. Also offer the gifts of the people and make atonement for them, as the Lord commanded Moses.”
8 Aaron therefore went to the altar and killed the young calf of the sin offering. 9 Then Aaron's sons brought the blood to him; and he dipped his finger in the blood, put it on the horns of the altar, and poured the blood at the base of the altar. 10 But the fat, the kidneys, and the fatty lobe from the liver of the sin offering he offered on the altar, as the Lord commanded Moses. 11 The flesh and the hide he burned with fire outside the camp. 12 Then he killed the burnt offering; and Aaron's sons presented to him the blood, which he poured all around on the altar. 13 Then they presented the whole burnt offering to him, with its pieces and head, and he put them on the altar. 14 He washed the stomach and the feet and put them on the altar for a whole burnt offering.
15 Then he brought the people's gift and took the young goat, which was the sin offering for the people, and killed it and offered it for sin, like the first one. 16 He also brought the whole burnt offering and offered it according to the prescribed manner. 17 Then he brought the grain offering, took a handful of it, and put it on the altar, besides the whole burnt offering of the morning. 18 He also killed the young bull and the ram as sacrifices of peace offerings, which were for the people; and Aaron's sons presented to him the blood, which he poured all around on the altar. 19 The fat from the young bull and the ram: the fatty tail, what covers the stomach and the kidneys, and the fatty lobe attached to the liver, 20 they put on the breasts. Then he offered the fat on the altar; 21 but the breasts and the right thigh Aaron removed as a choice portion before the Lord, as the Lord commanded Moses.
22 Then Aaron lifted his hand toward the people, blessed them, and came down from offering the sin offering, the whole burnt offerings, and peace offerings. 23 After this, Moses and Aaron went into the tabernacle of testimony, and came out, and blessed all the people. Then the glory of the Lord appeared to all the people; 24 and fire came out from the Lord, and consumed the things on the altar, both the whole burnt offerings and the fat. When all the people saw this, they were amazed and fell on their faces.
(Leviticus 9:1-24 OSB)
Thus, GOD showed His approval that Aaron and his sons were indeed GOD's appointed priests and that they had acceptably followed His instructions. The amount of sacrifices and the nature of each—that no one animal sacrifice was sufficient for all needs—should impress upon us the onerous nature of the Levitical priestly service under the Law and the severity of humanity's need for something—some ... One—better.