Well I can. I can lock my house doors from inside, without using key.
Edit
AAAAAAAAARRRRRRRRRRGHHHHHHH
Damn you, Inspector Anime!
Sill. We can't trust chapel opening scene; nor in anime, nor in novel. Battler not witnessed it. Nor he inspected chapel's door. Still, key being something abstract, should work. No matter how much people memorize key, there is still one key. And that key can be sealed in piece of paper.
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