Jesus sent this message to a church. It is a commendation for doing well. The image of an open door which cannot be shut tells us Jesus has all power and authority over this entrance and over who is allowed to walk through it. This group of people who have been faithful are invited to look forward to that open door He has prepared for them. It is a door not likely to be found on earth but in heaven. And there is another verse which tells us it is in heaven.