Jesus promised that He would return, and
His disciples were warned to watch for Him.

"And take heed to yourselves, lest at any
time your hearts be overcharged with
surfeiting, and drunkenness, and cares of
this life, and so that day come upon you
unawares.
For as a snare shall it come on all them that
dwell on the face of the whole earth.
Watch ye therefore, and pray always, that ye
may be accounted worthy to escape all these
things that shall come to pass, and to stand
before the Son of man."
Luke 21:34-36