"I set before thee an open door, and no man
can shut it".

The open door refers to the rapture of the
church. The promise of the rapture is given
to the Philadelphia Church.

In chapter 4 of revelation, "a door opened
in heaven". At the sound of the trumpet a
voice said, "come up
hither",
and
John was caught up
into heaven
representing the
rapture of the Church.