Avoiding Uncleanness
The Hebrew people were chosen by God to know Him and to serve Him. The tribe of Levi was chosen especially to serve God by serving various functions in the tabernacle and later in the temple. And the sons of Aaron were chosen by God to alone preserve the lamp, make sacrifices on behalf of the people, and enter (once per year) into the Holy of Holies -- the dwelling place of God.
The separateness, and holiness, that God required his priests to maintain is apparent from the first account of their ordination in Exodus 29:
35 "Do for Aaron and his sons everything I have commanded you, taking seven days to ordain them.
36 Sacrifice a bull each day as a sin offering to make atonement. Purify the altar by making atonement for it, and anoint it to consecrate it.
37 For seven days make atonement for the altar and consecrate it. Then the altar will be most holy, and whatever touches it will be holy.
36 Sacrifice a bull each day as a sin offering to make atonement. Purify the altar by making atonement for it, and anoint it to consecrate it.
37 For seven days make atonement for the altar and consecrate it. Then the altar will be most holy, and whatever touches it will be holy.
The sons of Aaron were kept apart from their people for seven days during the period of their ordination during which they were purified with water and the blood of animals. [NB: this method of purification -- i.e. time/blood/water/fire -- will be explored more deeply in the context of Purgatory] Likewise, the altar upon which sacrifices are offered to God must be made holy before use.
The following demands were made of God's priests -- over and above those prescribed for the layman Leviticus 21:
1 The LORD said to Moses, "Speak to the priests, the sons of Aaron, and say to them: 'A priest must not make himself ceremonially unclean for any of his people who die,
2 except for a close relative, such as his mother or father, his son or daughter, his brother,
3 or an unmarried sister who is dependent on him since she has no husband—for her he may make himself unclean.
4 He must not make himself unclean for people related to him by marriage, [a] and so defile himself.
5 "'Priests must not shave their heads or shave off the edges of their beards or cut their bodies.
6 They must be holy to their God and must not profane the name of their God. Because they present the offerings made to the LORD by fire, the food of their God, they are to be holy.
7 "'They must not marry women defiled by prostitution or divorced from their husbands, because priests are holy to their God.
8 Regard them as holy, because they offer up the food of your God. Consider them holy, because I the LORD am holy—I who make you holy.
...
10 "'The high priest, the one among his brothers who has had the anointing oil poured on his head and who has been ordained to wear the priestly garments, must not let his hair become unkempt or tear his clothes.
11 He must not enter a place where there is a dead body. He must not make himself unclean, even for his father or mother,
12 nor leave the sanctuary of his God or desecrate it, because he has been dedicated by the anointing oil of his God. I am the LORD.
13 " 'The woman he marries must be a virgin.
14 He must not marry a widow, a divorced woman, or a woman defiled by prostitution, but only a virgin from his own people,
15 so he will not defile his offspring among his people. I am the LORD, who makes him holy.' "
We see in this passage a difference in the holiness of a regular priest and that of the high priest. For a priest may become unclean and mourn the passing of a close relative, but not a non-relative or a relative by marriage. The high priest must never rend his clothes in mourning or visit the dead -- not even at the death of his parents. Again a regular priest must not marry a prostitute or a divorcee, but the high priest must marry only a virgin. The special holiness demanded of the high priest is due to his special closeness to God, whom we have seen is the source and summit of holiness.
Spiritual Uncleanness
I have so far limited my discussion to physical uncleanness. It serves my purpose in illustrating the need for holiness in those who would serve God, but it only tells half of the story. Exclusive focus on external cleanliness without an attempt to purify our inner selves is the sin of the Pharisee (cf. Luke 11). We see that for the priests of God, the punishments for violating the holiness of God through sin and disobedience could be severe -- even unto death.
We see in Exodus 10 the punishment visited upon Aaron's eldest sons:
1 Aaron's sons Nadab and Abihu took their censers, put fire in them and added incense; and they offered unauthorized fire before the LORD, contrary to his command.
2 So fire came out from the presence of the LORD and consumed them, and they died before the LORD.
3 Moses then said to Aaron, "This is what the LORD spoke of when he said: "'Among those who approach me I will show myself holy; in the sight of all the people I will be honored.'" Aaron remained silent.
Their disobedience in performing their priestly ministry was an affront to the holiness of God (cf. Acts 5). Simple disobedience was so abhorrent in those God had made holy (through their purification and ordination) that they were found deserving of death.
In the next section, we will look at more deeply into incompatibility of God's holiness with sin and uncleanness.