Why is a lamb standing before the Father as though it is slain?

Lutero:

I don't care what's contained in your pagan ritual. It has nothing to do with the issue at hand.  I asked  for scriptural reference to support his allegation that Jesus is perpetually offering Himself to His Father in heaven. I'm still waiting.

It is many places. But it is spiritually discerned. 

Ask yourself, why is a lamb standing before the Father as though it is slain? He is being offered to the Father.
Who can offer a lamb in heaven? Only Christ, the Lamb of God.
What does a slain lamb represent? The Passover Lamb offered to God in remembrance of Israel's salvation.

Revelation 5:6

King James Version (KJV)
⁶ And I beheld, and, lo, in the midst of the throne and of the four beasts, and in the midst of the elders, stood a Lamb as it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent forth into all the earth.

This is my answer to the question. Note that St. John wrote this book many years after Jesus had already been crucified. And the Lamb of God continues to appear in heaven before the Father. Therefore, Jesus, the Lamb of God, perpetually offers Himself to the Father in our behalf.

Sincerely,

De Maria