"When you hear that we look for a kingdom, you suppose, without making any inquiry, that we speak of a human kingdom; whereas we speak of that which is with God" (Justin - First Apology, 11).
Be on your guard! Stay awake! This encouragement you hear from Jesus today. And you might ask: Why should I be on my guard? Why am I to stay awake! What should I wait for! The answer is given by saint Justin Martyr. As a Christian you are supposed to look for a kingdom of God. Not in the non-specified future but now. You are a traveller on your way to the heavenly kingdom. And your task is to recognize Christ travelling with you. Perhaps He was hidden from your eyes in the person you passed today. If you could not see Him with your eyes, how will you be able to see Him with your heart? So stop during this Advent and pay attention in the dawn, noon, evening and night. A tip for you from the Gospel: He comes unexpectedly (Mc 13:37).