No graffitti & don't wash up in the little sink with water in it. Don't interrupt people in prayer just to introduce yourself. Don't walk up on the alter area & handle the icons.

Whatever you lick you get to keep.

Every cathedral I have ever been in wants you to take topless selfies and send them out into the world. At least that's what Jesus would do!

What the previous comments have said, plus BE QUIET!! If you need to talk, do it very softly. If there’s a service going on, DON’T walk across the front of the Nave. They usually don’t let tourists in during mass, but there may be matins, evensong, or benediction going on while tourists are there.

Just be respectful. I been to loads. (Liverpool has 2) including the church of the holy sepulchre in Jerusalem. that was weird.

Don’t shout “Hitchens Rules” to test out the acoustics?
Seriously though the Sacre Coeur in Paris is breathtaking, particularly when, as they were when I visited, nuns are singing in the crypt and a ray of sunshine from a high window strikes the gold painted part of one of the murals. As others have said, appreciate the beauty, majesty and skill of man’s creation.
Then wonder as I do why and how people put so much work and money and effort into celebrating something that is so obviously made up!

Dont touch!

I teach art but I've only been east of the Mississippi twice. I wish I was there!