The Book of Revelation mentions three doors:

Two are open doors:

Revelation 3:7  And to the angel of the church in Philadelphia write; These things saith he that is holy, he that is true, he that hath the key of David, he that openeth, and no man shutteth; and shutteth, and no man openeth;

8  I know thy works: behold, I have set before thee an open door, and no man can shut it: for thou hast a little strength, and hast kept my word, and hast not denied my name.
 
Revelation 4:1  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.
 
One is a shut door before which the Lord stands, knocking.
 
Revelation 3:20  Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.
 
Jesus Himself is The Door.
John 10:7  Then said Jesus unto them again, Verily, verily, I say unto you, I am the door of the sheep ...
9  I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture.
 
It is clear that no one can go through the open doors of Revelation 3:8 and Revelation 4:1, unless he first has opened the door of his heart, mentioned in Revelation 3:20, to Him who said "I am the Door".