"After this I looked, and, behold, a door was
opened in heaven: and the first voice which I heard was as it
were of a trumpet talking with me; which said, Come up hither, and I
will shew thee things which must be hereafter." (Rev. 4:1)
this" means, "After a person goes through all the stages described
in chapters 2 and 3."
door was opened" means, "The door of all-pervasive understanding and
realization will be opened to the person who has reached this
the first voice which I heard" is the voice of truth that comes from
Holy Ghost, which
guides a human to full realization.
which must be hereafter" means that only such a person will fully
realize and understand the things written in the rest of the book
The Revelation and their significance.