The parable of the doorkeeper is found in Mark's Gospel, chapter 13:34-36.
"[For the Son of man is] as a man taking a far journey, who left his house, and gave authority to his servants, and to every man his work, and commanded the porter to watch.
" Watch ye therefore: for ye know not when the master of the house cometh, at even, or at midnight, or at the cockcrowing, or in the morning:
" Lest coming suddenly he find you sleeping."
Scholars regard Mark's gospel as the earliest. This parable, which is reworked slightly in Luke's gospel in chapter 12:35-38, is easy to render in existential terms. Wakefulness, the need to ever present in every moment as a sign of something like authenticity is the rendering that springs most quickly to mind. But this reading almost seems trite.
Of course, early Christians expected Jesus to return. Recall Paul's hopes in his letters for an immediate return. Is this simply an exhortation to be remain alert lest one is caught unawares? This seems to literal to satisfy. The second coming has been an awfully long time in coming.
I prefer a reading that stresses wakefulness, and the rejection of complacency. A spirit alive senses change in every wind. Although I can say I have any comprehension of the divine, I am aware that at root we are such things as dreams are made of. We can render the world in safe terms, and live within the confines of concepts to which lend a false sense of reality. Or we can accept the fact that ever the readiest anchor is incapable of tethering us to a reality so stable as to eliminate the need for faith and hope in what is beyond our ken.
Is it possible that the Kingdom of God, immanent yet forever removed, is really nothing more than humility in the face of what cannot be known, a humility that requires not just faith and hope, but charity as well? Recall that the greatest gift is love; perhaps it is also the most necessary gift of all.