‘And Yahweh said to Moses, “Speak to Aaron your brother, that he come not at all times into the holy place within the veil, before the mercy-seat which is on the ark, that he die not. For I will appear in the cloud on the mercy-seat.” '
God's first warning is that Aaron recognise that the High Priest does not have the right to enter the Holy of Holies, ‘the holy place within the veil', except by strict permission, that permission being given only on the Day of Atonement. He does not have unrestricted access. For while God appears in the cloud on the mercy-seat, man may approach Him, apart from on the Day of Atonement, only from the other side of the veil. He cannot enter the throne room. To approach the mercy-seat direct could only be a once in the year experience. ‘The Holy Spirit signifying this, that the way into the Holy of Holies was not yet opened up, while the first tabernacle was still standing' (Hebrews 9:8). The veil said, thus far shall you come and no further.
The cloud was presumably the cloud that had accompanied Israel from Egypt, the cloud of His presence which by night became a fire (Exodus 13:21-22 and often).
We are reminded here of how when God revealed His glory on the face of Moses the people were afraid to come near him, and he had to veil his face. None but Moses could cope with the glory of God, until One came whose face also shone like the sun revealing His Father's glory (Matthew 17:2; John 1:14; John 1:18). Thus the need for the veil and the cloud.
The ‘propitiatory' or mercy-seat was the covering on the Ark of the Covenant of Yahweh, where atonement could be made and man become reconciled to God. It was the ‘kapporeth', literally the place of propitiation, the place where reconciliation and atonement was finally performed. This was a solid gold slab on which were the two cherubim at either end looking inward. It was the same size as the chest which it covered. It comes from the root ‘kpr' (to cover) and the conjugation used signifies the place where sins are ‘fully covered' so that they are no longer seen by God and held against the sinner (Jeremiah 18:23). It is the place of propitiation and expiation, the place where the punishment for sin was met by the shedding of blood, the place of atonement, of reconciliation, where He and His people were made at one. There is also a suggestion behind it that it is the earthly throne of Yahweh between the cherubim.
The writer to the Hebrews in the New Testament very much has this Day in mind in Hebrews 9-10, seeing its real fulfilment in the offering up of Jesus Christ on our behalf by Himself as our great High Priest. That once-for-all offering of Himself would replace for ever this Day of Atonement, and all the other offerings, sacrifices and rituals of this earthly tabernacle.