It is a solemn warning to be alert at all times. Hence the lies that the guards told at the time of our Lords resurrection. When Jesus was going through that agonising time in the garden of Gethsemane, he repeatedly came and found the disciples asleep.
Many today are spiritually asleep.
In the whole time span of human history, there has never been a time such as this. We need to awake out of our sleep. Christians need not be taken by surprise at our Lords coming.