1. It is the perfect symbolism of what one would expect where Heaven meets Earth. There is always a picture of Mary and Jesus. The Theotokos of course is Jacob's Ladder so to speak, God's connecting point between Heaven and Earth. Jesus is of course true God and true Man. But also, there are the saints, the holy ones of God who though human abide in Heaven.
2. The Incarnation is front and center. The middle doors (Royal Doors) always portray the moment of Gabriel anouncing to Mary that she will bear the Christchild. On either side of the Royal Doors is a portrayal of Christ and an icon of Mary holding Jesus. The Incarnation is the removal of that which separated Heaven and Earth (see above).
3. The opening of the curtain highlights the removal of barriers between Heaven and Earth. The curtain is opened at key moments symbolizing the Revelation of God (readings, Eucharist, etc). It is a beautiful and subtle reminder that man had walked in darkness, but no more.
4. Like the name Divine Liturgy itself, we are made to remember that we are worshipping not just on Earth. I read a book called Lamb's Supper when I was in the seminary. I remember that Scott Hahn pointed out that the Mass is occuring in Heaven as well as Earth and that we are constantly worshipping alongside angel's in the same liturgy described in Revelation and OT prophetic writing. At the time that blew me away because I couldn't remember anyone teaching this or even talking about it. Then I came to the Melkite parish where I still am today and found out that Eastern Churches call it Divine Liturgy instead of Mass and I was hooked.
May God bless all who read my ramblings,
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