The first words of the fourth chapter of
Revelation, "hereafter", indicates that
events now taking place, are after the
Church Age. The event which ended
the Church Age, as taught in the New
Testament is the rapture.

We read that "a door was opened in
heaven." A promise had been given to
the Philadelphia Church of "an open
door". (Revelation 3:8)

And this open door is associated in
the verses that follow, with the promise
that He would keep the Philadelphia
Church "from the hour of temptation"
[the Great Tribulation].

These words of Christ are most surely
speaking of the rapture.