Numbers 18:3-4

3They shall keep guard over you and aover the whole tent, bbut shall not come near to the vessels of the sanctuary or to the altar clest they, and you, die. 4They shall join you and keep guard over the tent of meeting for all the service of the tent, dand no outsider shall come near you.

2 Kings 23:9

9 eHowever, the priests of the high places did not come up to the altar of the Lord in Jerusalem, but they ate unleavened bread among their brothers.

1 Chronicles 23:27-28

27For by the last words of David the sons of Levi were numbered from ftwenty years old and upward. 28For their duty was to assist the sons of Aaron for the service of the house of the Lord, having the care of the courts and the chambers, the cleansing of all that is holy, and any work for the service of the house of God.

Ezekiel 40:46

46and the chamber that faces north is for the priests gwho have charge of the altar. These are hthe sons of Zadok, who alone
Hebrew lacks  alone
among the sons of Levi may come jnear to the Lord to minister to him.”

Ezekiel 44:12-15

12Because they ministered to them before their idols and became ka stumbling block of iniquity to the house of Israel, therefore I have lsworn concerning them, declares the Lord God, and they shall bear their punishment. 13They mshall not come near to me, to serve me as priest, nor come near any of my holy things and the things that are most holy, but nthey shall bear their shame and the abominations that they have committed. 14Yet I will appoint them to keep ocharge of the temple, pto do all its service and all that is to be done in it.

Rules for Levitical Priests

15“But qthe Levitical priests, rthe sons of Zadok, who kept sthe charge of my sanctuary twhen the people of Israel went astray from me, shall come near to me uto minister to me. And they shall stand before me to offer me vthe fat and the blood, declares the Lord God.
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