...are they atheist aliens?
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I would likely say to them that the church represents a body, the one that the church makes up. It's a skeletal structure, considering that we're looking at an old gothic church. It's the framework in the flesh. At the heart of it, the eucharist and the wine. Transubstantiation 'makes' a body a continuous cycle that envelopes that skeletal structure (conjunction of earth and spirit = the cross). I would also say that it was placed over a place on earth that coincides with points in the sky. I would tell them that the bell tower represents the pineal gland, and when 'church is in', the bell had rung. It would be beneficial esoterically to point out that the church was build on proportions that suit the book of Revelations by craftsmen. 4 of those craftsmen make up the 'four corners'.
Followed by a long-winded jaunt into particle territory. Adam and the 'rib'. They'd laugh, we'd vaporize a beer.
Fin, Fin