Make an atonement cover of pure gold—
two and a half cubits long and a cubit and a half wide.
And make two cherubim out of hammered gold
at the ends of the cover.’

Exodus 25:17-18

The lid of the chest was properly the atonement cover, though William Tyndale (died 1536) coined the meaningful expression ‘mercy seat’. On it was yearly sprinkled the blood of the sin offering for the people. The cherubim of glory were formed as one with the cover, looking down upon the blood.
All was a portrayal of the once-for-all offering of Christ.

Daily Reading: 1 Samuel 26