In the Catholic Mass, the Word is “unsealed” for us through its preaching by the priest, or deacon, who wear a white garment because they are joined to the Lamb in a special way through Ordination. The Faithful then come before an altar superior to the first and sing “holy, holy, holy Lord God of power and might…” after which they kneel down so that when the Lamb becomes present in the bread & wine they may imitate the posture of those in God’s heavenly court. Sometimes incense is also utilized, symbolizing prayers rising up to and pleasing God.
So what does all of this have to do with the Mass? The Mass is a participation in the worship of God in the Heavenly Temple. Without Holy Thursday, Good Friday, and Easter Sunday, none of the vision Saint John reports is real. With them on the other hand, the “slain Lamb” becomes the One crucified on Good Friday, who lives despite his wounds because of His Resurrection on Easter Sunday. The new Divine Ritual for perpetuating these realities was given Holy Thursday night, when Jesus told us that the Lamb would be made present in bread and wine.