From: Luke 17:20-25
The Coming of the Kingdom of God
 Being asked by the Pharisees when the Kingdom of God was coming, He (Jesus) answered them, "The Kingdom of God is not coming with signs to be observed;  nor will they say, `Lo, here it is!' or `There!' for behold, the Kingdom of God is in the midst of you."
The Day of the Son of Man
 And He said to His disciples, "The days are coming when you will desire to see one of the days of the Son of Man, and you will not see it.  And they will say to you, `Lo, there!' or `Lo, here!' Do not go, do not follow them.  For as the lightning flashes and lights up the sky from one side to the other, so will the Son of Man be in His day.  But first He must suffer many things and be rejected by this generation."
20-21. Like many Jews of their time, the Pharisees imagined the establishment of the Kingdom of God in terms of external, political authority; whereas Jesus teaches that it is something eminently spiritual, supernatural, which has been happening since Jesus' coming, although its climax will be after His Second Coming or Parousia at the end of the world; its effect is to be seen, above all, in men's hearts, although it is also something visible and external, just as the Church has a visible dimension.
The presence of the Kingdom of God in each soul is something one perceives through the affections and inspirations communicated by the Holy Spirit. St. Therese of Lisieux says this about her own experience: "The Doctor of doctors teaches us without the sound of words. I have never heard Him speak, and yet I know He is within my soul. Every moment He is guiding and inspiring me, and, just at the moment I need them, `lights' till then unseen are granted me. Most often it is not at prayer that they come but while I go about my daily duties" ("The Story of a Soul", Chapter 8).
22. After the Apostles receive the Holy Spirit on the day of Pentecost they will devote their whole lives to preaching boldly the message of Jesus Christ, and winning all people over to the Lord. This will lead them to experience many severe contradictions; they will suffer so much that they will yearn to see even "one of the days of the Son of Man", that is, one of the days of the victory of Jesus Christ. But this day will not arrive until the Lord's Second Coming.
23-36. These words of our Lord are a prophecy about the last coming of the Son of Man. We should remember that prophecy often involves events on different levels, many symbols, a terminology of its own; the "chiaroscuro" which they create gives us insight into future events, but the concrete details only become clear when the events actually occur. Our Lord's last coming will be something sudden and unexpected; it will catch many people unprepared. Jesus illustrates this by giving examples from sacred history: as in the time of Noah (cf. Genesis 6:9-19:7) and that of Lot (cf. Genesis 18:16-19:27) divine judgment will be visited on men without warning.
However, it is useful to recall here that everyone will find himself before the divine Judge immediately when he dies, at the Particular Judgment. Thus Jesus' teaching has also a present urgency about it: HERE AND NOW a disciple should scrutinize his own conduct, for the Lord can call him when he least expects.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland.
Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.