What a great space. There are some photos of religious-themed rooms on houzz as reference.

I'm not a designer, but I wonder if the scale of the low table is right? It seems like a console table with more height would be in proportion to the high ceiling. Also, I've read that most make the mistake of hanging pictures and mirrors too high, and that the general "rule" of thumb is eye level when seated. I like the floor lamps on either side of the sofa and the idea of furniture facing the visitor upon entry - seems welcoming; but again, I wonder about scale - would 2 smaller chairs be in better proportion with the width of that space centered around a small chest in between? Finally, I've read that in rooms with very high ceilings, the room becomes grounded with a boldly patterned rug. I can't tell about the flooring on my screen.

Another reference I'd like to share that may be of interest is one for gorgeous foyers in Jan Showers' book, Glamorous Rooms. Wow factor especially in spaces as large and wonderful as this one! This space seems to lend itself to grandeur and her version is very understated suitable for a sacred place such as this.

Damien, How wonderful that you are preserving and using this beautiful convent. I would lower the gold mirror to eye level and would caution you to not overuse highly ornate and gilded surfaces so that the architectural beauty and plaster details are not upstaged.

I can't read the floors from the picture but I like them dark. My only suggestions would be lamp shades with color and same for stair runner. Two great opportunities to break up the gold tones. Nice!

At first glance, this is beautiful... but here are my thoughts after looking more closely. The oval mirror is either too small (try sconces on either side) or the table below (that is also crammed into the corner) is too short.
The dark pedestal piece at the rear of the hall beneath the stairs is too narrow and the artwork/vase on top seems exceptionally weighty against the wall with the dark mirror. Something needs to change back there... all 3 pieces need to be broken up with a cream or gold.
Also, the flooring is very dark - I'd like to see a lighter rug on the stair (I can understand that you'll want to consider traffic patterns and upkeep with a bustling guest house, so you may want to try a lighter area rug on the main level?
What a delight it must be to have such an exquisite canvas!