In recently preaching on Matthew 5:7, “Blessed are the merciful, for they shall obtain mercy,” I was overwhelmed with the references to “mercy” in the Bible. I found 359 occurrences of “mercy” in its various forms. Of these 359 occurrences, 56 of these are describing the “mercy seat”. The first usage of this term is found in Exodus 25:17: “You shall make a mercy seat of pure gold; two and a half cubits shall be its length and a cubit and a half its width.” The context of this verse is the description of the ark of the testimony (vv. 10-22). This was located in the Holy of Holies in the tabernacle.
Here is an artist’s rendition of the ark of the testimony (or covenant), with the cherubim on each end of the mercy seat, which is the cover of the ark.
https://www.biblestudytools.com/bible-study/topical-studies/what-is-the-mercy-seat-and-why-is-it-important.html
The instructions for the Day of Atonement can be found in Leviticus 16. Verses 11-17 describe how once a year only the high priest could enter the Holy of Holies to make atonement (covering, reconciliation) for his sin and the sins of the people by sacrificing and then sprinkling the blood of a bull and of a goat, respectively, on the mercy seat.
We know from Hebrews 10:4 that it is not possible that the blood of bulls and goats could take away sins. The book of Hebrews speaks of the superiority of Christ. The Old Testament sacrifice pointed to the once-for-all sacrifice of Jesus Christ on our behalf. Hebrews 9:1-10 describes the earthly sanctuary and explains that it was symbolic for that present time. Then, in verses 11-15, the writer of Hebrews goes on to give the fulfillment of this picture in Christ, our great High Priest (Heb. 4:14-16). This is worth taking the time to read:
“But Christ came as High Priest of the good things to come, with the greater and more perfect tabernacle not made with hands, that is, not of this creation. Not with the blood of goats and calves, but with His own blood He entered the Most Holy Place once for all, having obtained eternal redemption. For if the blood of bulls and goats and the ashes of a heifer, sprinkling the unclean, sanctifies for the purifying of the flesh, how much more shall the blood of Christ, who through the eternal Spirit offered Himself without spot to God, cleanse your conscience from dead works to serve the living God? And for this reason He is the Mediator of the new covenant, by means of death, for the redemption of the transgressions under the first covenant, that those who are called may receive the promise of the eternal inheritance.”
May we daily rejoice in His mercy and atonement for us sinners, whereby He makes us saints!