What was the Shekinah glory on the mercy seat in the tabernacle?
The Shekinah glory upon the mercy seat was the very presence of God, appearing in the cloud upon the mercy seat that rested on the ark behind the veil. The high priest could enter that place only once a year, on the Day of Atonement, because Old Testament sins were covered, not atoned.
Les explains that the Shekinah glory rested upon the mercy seat behind the veil, and that this was the very presence of God. In the Day of Atonement chapter, the Lord spoke to Moses after the death of Aaron's two sons, warning Aaron not to come at all times into the holy place behind the veil, before the mercy seat upon the ark, lest he die. God said He would appear in the cloud upon the mercy seat. Because Old Testament sins were not atoned for but only covered, the believer could not enter God's presence, and even the high priest could go behind the veil only once a year on the Day of Atonement. Les also notes the priest had to undergo cleansing, just as the Passover lamb had to be perfect without blemish, and Christ fulfilled these pictures when He went to the cross.
And the LORD said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the vail before the mercy seat, which is upon the ark; that he die not: for I will appear in the cloud upon the mercy seat.