Leviticus 16:2

And the Lord said unto Moses, Speak unto Aaron, thy brother, that he come not at all times, at any time that he might choose, into the Holy Place within the veil, into the Most Holy Place, before the mercy-seat, which is upon the ark, the corer, or lid, of the chest which contained the tables of the covenant, that he die not; for I will appear in the cloud upon the mercy-seat. That was the place appointed for the peculiar manifestation of God in the glory of His mercy, Exodus 25:22; for this cloud revealed the presence of God to the representatives of the people. To step into this place of highest consecration in an arbitrary way, without the special direction of God, would result in death to the offender.