The Mercy Seat:
17 “You shall make a mercy seat of pure gold. Two cubits and a half shall be its length, and a cubit and a half its breadth. 18 And you shall make two cherubim of gold; of hammered work shall you make them, on the two ends of the mercy seat. 19 Make one cherub on the one end, and one cherub on the other end. Of one piece with the mercy seat shall you make the cherubim on its two ends.
20 The cherubim shall spread out their wings above, overshadowing the mercy seat with their wings, their faces one to another; toward the mercy seat shall the faces of the cherubim be. 21 And you shall put the mercy seat on the top of the ark, and in the ark you shall put the testimony that I shall give you. 22 There I will meet with you, and from above the mercy seat, from between the two cherubim that are on the ark of the testimony, I will speak with you about all that I will give you in commandment for the people of Israel. Exodus 25:17-22 (ESV)
God was above the ark of the covenant. The ten commandments in the ark could not save but only condemn the people. The lid of the ark or atonement cover was called the mercy seat.
The mercy seat was only put into use once a year on the Day of Atonement.
11 “Aaron shall present the bull as a sin offering for himself, and shall make atonement for himself and for his house. He shall kill the bull as a sin offering for himself.
12 And he shall take a censer full of coals of fire from the altar before the LORD, and two handfuls of sweet incense beaten small, and he shall bring it inside the veil 13 and put the incense on the fire before the LORD, that the cloud of the incense may cover the mercy seat that is over the testimony, so that he does not die. 14 And he shall take some of the blood of the bull and sprinkle it with his finger on the front of the mercy seat on the east side, and in front of the mercy seat he shall sprinkle some of the blood with his finger seven times. 15 “Then he shall kill the goat of the sin offering that is for the people and bring its blood inside the veil and do with its blood as he did with the blood of the bull, sprinkling it over the mercy seat and in front of the mercy seat. 16 Thus he shall make atonement for the Holy Place, because of the uncleannesses of the people of Israel and because of their transgressions, all their sins. And so he shall do for the tent of meeting, which dwells with them in the midst of their uncleannesses. Leviticus 16:11-16 (ESV)
The Bible often describes the work of Christ on the cross in terms of the mercy seat or propitiation.
17 Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation for the sins of the people. Hebrews 2:17 (ESV)
23 for all have sinned and fall short of the glory of God,
24 and are justified by his grace as a gift, through the redemption that is in Christ Jesus, 25 whom God put forward as a propitiation by his blood, to be received by faith. This was to show God’s righteousness, because in his divine forbearance he had passed over former sins. Romans 3:23-25 (ESV)
2 He is the propitiation for our sins, and not for ours only but also for the sins of the whole world. 1 John 2:2 (ESV)
In these verses, the atoning sacrifice or propitiation (hilasterion) refers directly to the blood sprinkled on the mercy seat.