Charles Spurgeon Commentary Exodus 25:17

"And thou shalt make a mercy-seat of pure gold: two cubits and a half [shall be] the length thereof, and a cubit and a half the breadth thereof." — Exodus 25:17 (ASV)

It fitted exactly on the top of the ark, and so completely covered whatever was put within. It was made of pure gold. This, perhaps, was the most important part of this very important item of tabernacle furniture. It was the mercy seat, the cover that hid the law, the place where God promised to meet with his people.