Revelation 5:6 - "a Lamb standing as if it had been Slaughtered, having Seven Horns and Seven Eyes,
which are the Seven Spirits of God sent out into all the Earth".
"What Wonderful Majesty! What Stupendous Condescension! O Sublime Humility! That the Lord of the Whole Universe, God and the Son of God, should Humble Himself like this, under the Form of a little Bread, for our Salvation"
-Saint Francis of Assisi
And what do the People do? They Rejoice, and they Break-out into a Song. And what is the Song, "Worthy art Thou to take the Scroll and to Open its Seals ,for Thou was Slain". Past Tense, "And by Thy Blood didst Ransom Men for God from every Tribe and Tongue and People and Nation". And what has He done? He's become a Priest to be sure, but for what Purpose? "He has made them a Kingdom and Priest to our God". He has made those whom He has Saved, Priests. And what do Priests do? They offer Sacrifice.
Has Christ's Sacrifice ended all Sacrifices? No!. Christ's Sacrifice has ended all Ineffective, Bloody, Animal Sacrifices that never did anything anyway. Now for the First Time in History, we can really begin to Offer Sacrifice to God. Romans 12 says, "Offer your Bodies as a Living Sacrifice, Holy and Acceptable to God. And it wouldn't be Holy and Acceptable, except that it's United to Christ's Perpetual Sacrifice. He's not Bleeding. He's not Dying. He's not Suffering, but He is Offering a Sacrifice as a Lamb does, as a Priest King does Continually, Forever.
And that's what it's all about. John wouldn't see a Lamb, looking as though it had been Slain, if the whole Kit and Caboodle was Completed and Done. Yes, it's Completed and Done, but it's still Going-on, and it's going to Go-on Forever, in the Future.
He is a Priest in Heaven, Ministering now in the Sanctuary, and He's got something to Offer, and He's Continually Offering it. He's just not Bleeding, and Dying, and Suffering any more. He's not Killing any more Animals, but He's Continually Offering the Once-and-for-All Sacrifice which is Himself; but it's a Continual Sacrifice. It's a Perpetual Offering. He's not Dying, but He's still Offering. That's exactly what the Catholic Church Teaches about the Mass.
Christ, through the Holy Spirit, makes Himself available as the Lamb of God, to be Consumed Continuously. That's the Whole Point of the Resurrection, Incidentally. The Holy Spirit Raises-up that Body and Glorifies it, so Supernaturally, that Body and Blood, which is Glorified, may be Internationally Distributed through the Elders and Priests of the Church, so that all of God's Children can be Bound-back to the Father in the New Covenant Sacrifice of Christ. He didn't Die again. He's not Bleeding and He's not Suffering. He's Reigning in Glory and Giving us His Own Glorified Flesh and Blood.