It's a beautiful image with deep meaning. The Lord will not force himself on anyone; he is waiting to enter our hearts but only if we invite him in. He respects our free will. It is up to us to get up and open the door.
But what about those who are trapped in addictions, paralyzed by despair? Those whose wounds keep them chained in pain? We've all been in a dark place ourselves, mired in our own sins or the consequences of the sins of others. In our shame and confusion, we hardly dare lift our eyes to heaven, let alone open our doors and expose our misery. What does Jesus do then? Does he still stand outside, knocking, waiting for us to pull ourselves up by our bootstraps and find the courage to open the door?
There's another image in the Gospel of John (the same author as Revelation). Imagine the Apostles in the Upper Room on Easter Night. They are huddled behind a closed door, and this one not only has no doorknob on the outside, it is chained, barred, and bolted from within. Maybe there is even some heavy furniture pushed in front of it. No one is going to be coming through that door! The Apostles are gathered together in fear, doubt, and confusion. Imagine their pain: not only are they afraid for their lives, but the very one in whom they have put their entire hope is now dead, shamefully tortured.
Luckily for them, Jesus does not stand outside knocking, waiting for an invitation to enter. A closed door is no obstacle for our God, not even a barred and bolted one. "Jesus came, although the doors were locked, and stood in their midst and said to them, 'Peace be with you.'" (John 20:26)
