On which door is Jesus knocking in Rev. 3:20?

We read in Revelation 3:20: "I stand at the door and knock ..." Jesus speaks in figurative terms te help the sinner to understand what it means to let the Saviour come into his life and heart. Jesus does not knock with his finger on a visible door of the human heart. No! He knocks by his Word, the Bible message when it is preached and the sinner realizes that God speaks to him personally. The sinner does not open an actual, visible door for the Lord to come in. He speaks to Jesus in prayer and ask him to enter into his life, to take away his sins and to rule over his life by the Holy Spirit. It is the same when you pray. You do not knock on a visible door. You "knock" when you speak to Him in prayer, then he hears you and answers your prayer (Matthew 7:7-8).