They had simply locked the world out in locking themselves in. We, like those first followers of Jesus, can be bound up and imprisoned, intimidated by the tasks and the world that faces us out there. Disease may threaten, or old age, or unemployment. A loss of job, the future of our children, heavy financial responsibilities all loom up out there. And it all isn't simply a lack of imagination, or merely a lack of nerve.
Life can in fact be difficult and quite dangerous.
What they feared wasn't due to a mere lack of imagination. Into that scene Jesus presented Himself to them. Do not be afraid. The real difficulty those first disciples faced was not so much what surrounded them as it was what was raging within them. The real enemy is not without, it is within.