Theophylact of Ohrid

He mentioned purification (the propitiatory sacrifice) and blood in order to convince the Jew that forgiveness and justification are accomplished through Christ. If, he reasons, you believed that sins were absolved by the blood of sheep, then all the more are they absolved by the Blood of Christ, and if the legal purification, being a type of Christ, had such power, then the truth itself has far greater power. The "purification" (mercy seat) was the cover of the ark, adorned with cherubim placed on both sides of it. …

Commentary on Romans ↗

Cosmas Indicopleustes

In the inner place was set the Ark of the Propitiation, which was concealed behind the veil, and was not seen by any one. The Propitiatory was a type of the Lord Christ according to the flesh, as saith the Apostle: Whom God set forth to be a propitiation by his blood; and again the high priest was himself a type of the Lord Christ, according to the Apostle: For, saith he, just as the high priest once a year entereth into the inner Tabernacle, so Christ, having come a high priest of the good things to come through his own blood, …

The Christian Topography, Book 5 ↗

Thomas Aquinas

Second, he shows whence this redemption has efficacy, when he says whom God has proposed to be a propitiation. For Christ's satisfaction was efficacious for justifying and redeeming by the fact that God had assigned him to this in his plan, which he designates when he says whom God has proposed to be a propitiation: according to the purpose of him who accomplishes all things according to the counsel of his will (Eph 1:11). …

Commentary on Romans ↗