"The door" is a figurative symbol of the means by which man "enters" and "exits" from one spiritual and moral state to another.

The "door" is a symbol of God's Will, His Commandments, through the knowledge and fulfillment of which man "enters" into a state of holiness, righteousness, and attains to Eternal Life.

"The Door" is also a symbol of the Teachings of Jesus Christ, by the fulfillment of which man enters into the knowledge of the Will of God, the benefits of a righteous life and entrance into Eternal Life are revealed to him.  

"Man's door" in its metaphorical meaning is his feelings, his mind, his conscience, through which he realizes his communication with the world around him and perceives or rejects the Truth of God's Word.