Doors do good things or bad things. They can allow you in or out; they can protect what's on either side of the door. Doors in zoos, for example, protect the animals inside from the people outside and vice versa.
They also restrict access: we read that when God drove Adam and Eve from the Garden, He placed cherubim and a flaming sword to secure the way to the Tree of Life. And when Noah finished the Ark and all of the animals were safely in, the Lord closed the door to the Ark; no one else was allowed in.
We live our lives behind doors; in the world we live in, we do not feel safe without doors and fences and gates with locks. There are places where people don't lock their doors; San Bernardino is not one of them. It used to be said that church doors were never locked; again, we have first-hand experience of why that doesn't happen here. It is a sign of the sinfulness of our society that churches experience arsonists and vandals and thieves. And this is nothing new: in 1453, when the Ottoman Turks were at the doors of Constantinople, which was at that time the political and religious head of the Eastern Orthodox churches, the priests and the people crowded into the magnificent church of Santa Sophia for safety. The doors were locked--and it was a trap.
Doors can be opportunities: we often face multiple doors. They may also be traps. (This has become the basis for TV game shows, where contestants are faced with several doors: one with something really valuable, the others less desirable. We can watch, and make our own guesses, and see which would have been best, with no risk to ourselves--and also no gain.) In our real lives, there are times where we can only choose one door; each choice takes all the alternates away. We choose schools, jobs, friends, spouses, residences, and more--and each one changes the next set of doors. Some choices open up more opportunities, some close down things we mighta-coulda-woulda done differently.
In Scripture, we find mention of doors separating the sacred from the ordinary. In tabernacle description in Exodus, we find doors between the various levels of sacred spaces, The Presence of God was believed to reside in the innermost chamber, with restrictions on who was allowed to pass through which doors and under what conditions. In Solomon's Temple and the rebuilt Temple of Jesus’ day, there was a similar series of enclosures with increasingly limited access, ending with the Holy of Holies, that was supposed to be entered only once a year by a specially chosen priest. It is significant, in the context of Jesus calling himself The Door, that the entrance to this area (which was a set of overlapping veils functioning as a door) was ripped open, top to bottom, at the time Jesus was crucified. And to make it more final, the Temple itself was destroyed less than 40 years later. As Christians, we see God opening the access to His Presence, through the ultimate sacrifice of the one who called Himself The Door.