“They will bring them, as the Israelites bring their grain offerings, to the temple of the LORD in ceremonially clean vessels.” Isaiah 66:20
The grain offering according to the Old Covenant was one of the forms of sin offering, and was made for the forgiveness of sins, and what is the grain offering, but the offering of bread, and what is the bread of life but the manna from heaven, and who is the manna from heaven but Christ our Lord? Every grain offering was a pointer to the one who not only forgives, but is Forgiveness. He is the Bread of Life, the one who is come down from heaven to be born in Bethlehem (which means ‘House of Bread’)
And do you see how the Israelites had to bring their grain offering in clean vessels? Who is the vessel for the grain offering but the Blessed Virgin–she who carried the Bread of Life in her own womb as the grain offering for the sins of the world. Here the Immaculate Conception is foreshadowed, for just as the grain offering had to be brought to the Temple of the Lord in a clean vessel, so the vessel of the Bread of Life had to be clean in order for the sacrifice to be acceptable.
(Biretta tip to Rob)