To which church did Jesus say, "I have placed before you an open door, which no one can shut"?

Answer: Philadelphia

Philadelphia had little strength but kept Christ's word, and He set before it an open door no one can shut.

“To the angel of the church in Philadelphia write: These are the words of the One who is holy and true, who holds the key of David. What He opens no one can shut, and what He shuts no one can open.”— Revelation 3:7 (BSB)