We learn from Leviticus 16:1 that Moses received instructions regarding the Day of Atonement, Yom Kippur, immediately after the judgment of Nadab and Abihu (ch10). Obviously he inserted chapters11-15 in the chronological narrative for a purpose. He probably did so because of the connection between the clean and unclean distinctions in these chapters and the emphasis on priestly purity that ended with the judgment of Nadab and Abihu (ch10). There is also continuity in the emphasis on the importance of holiness when entering the presence of Yahweh (ch16). [source][source][source]
As usual, God revealed these laws to Moses, not directly to the priests or even the high priest, Aaron ( Leviticus 16:2). Moses was the great mediator between God and the Israelites superior even to the high priest. Moses served in the role of a prophet when he did this. Later in Israel"s history, the prophets continued to communicate instructions from God not only to the priests but also to the kings. [source][source][source]
Even the high priest was not to enter the presence of God in the holy of holies, symbolized by the cloud over the mercy seat, at any time. If he did, he would die, as Nadab and Abihu did. What follows is instruction about when and how he could enter. The only way anyone could approach God when He manifested Himself on Mt. Sinai ( Exodus 19) was also as He specified. God was just as holy and demanded just as much reverence when He was dwelling among His people as when He dwelt away from them. Now He dwells within each Christian ( John 14:17; Romans 8:9; 1 Corinthians 12:13). [source][source][source]