The warning not to enter the Holy Place was a general rule with only one exception. The High Priest alone could enter the sacred area only on the Day of Atonement. The reason Aaron may not enter the innermost sanctuary at any time is that it houses the ark on which the mercy seat is found. It is there that God manifests his presence to his people. Before the tabernacle had been built God had come to his people on Mount Sinai. Now he dwells among them in the innermost part of the sanctuary. These laws highlight the truth that God is just as holy and demands just as much reverence when he dwells permanently with Israel as on the first occasion when he appeared on Sinai.
2 and the LORD said to Moses, “Tell Aaron your brother not to come at any time into the Holy Place inside the veil, before the mercy seat that is on the ark, so that he may not die. For I will appear in the cloud over the mercy seat.