The Ark of the Covenant or "Ark of the Testimony" was the only article of furniture in the Most Holy. (Hebrews 9:2-4) The word testimony suggests that it illustrated Jehovah's plan. It represented the eternal purpose of God–his foreordained arrangement of grace for mankind in the Christ (Head and Body). It therefore represented Christ Jesus and his Bride, the “little flock.”

The Ark was a rectangular box overlaid with gold. It contained the two Tables of the Law (Deuteronomy 31:26), Aaron's Rod that budded (Numbers 17:8), and the Golden Pot of Manna (Exodus 16:32). The Law showed how the Christ would meet in full all the requirements of God's perfect Law, and also that legal authority would be vested in him as the Law-executor.

Within the golden Ark was represented the glory to be revealed in the divine Christ: in the budded rod (God’s chosen priesthood), in the tables of the Law (the righteous Judge), in the incorruptible manna in the golden bowl (immortality, the divine nature). 

Above this Ark, and constituting a lid, was "The Mercy Seat"–a slab of solid gold, on the two ends of which were formed two cherubim. These cherubim had wings uplifted as if ready to fly, their faces looking inward toward the center of the plate on which they stood. Between the cherubim, on the "Mercy Seat," there was believed to be a bright, supernatural light representing Jehovah's presence. 

The light, called the Shekinah glory, represented Jehovah himself as the Light of the universe. (Similarly, Christ is the Light of the world.) This is abundantly testified by many scriptures. "Thou that dwellest between the cherubim, shine forth." (Psalm 80:1; 1 Samuel 4:4; 2 Samuel 6:2; Isaiah 37:16)

As the Ark represented the Christ, so the "Mercy Seat," (glory-light and cherubim together) represented Jehovah God–"the Head of Christ is God." (1 Corinthians 11:3)

The slab of gold called the "MERCY SEAT" (or Propitiatory, because on it the Priest offered the blood of the sacrifices which propitiated or satisfied the demands of divine justice) represented the underlying principle of Jehovah's character–justice. God's throne is based or established upon Justice. "Righteousness and justice are the foundation of thy throne." Psalm 89:14; Job 36:17; 37:23; Isaiah 56:1; Revelation 15:3

The Apostle Paul uses the Greek word for Mercy Seat or Propitiatory (hilasterion) when referring to our Lord Jesus, saying–"Whom God hath set forth to be a Propitiatory

[or Mercy Seat]…to declare his righteousness…that he might be just and the justifier of him which believeth in Jesus." (Romans 3:25,26) It pleased God that in his well-beloved Son, our Lord Jesus, all of His own fullness should dwell, and be represented to mankind. 

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