In this parable, a king arranges a marriage feast for his son. The king, of course, is God the Father and the son is Jesus Christ our Lord, while the feast signifies eternal life in God's heavenly Kingdom. Those who are initially invited to this feast are the Jews, God's chosen people. Most of them, however, do not take the invitation seriously. Instead, they make light of it, while some go so far as to persecute and even to kill the prophets. For these ungrateful people, it was "business as usual:" the status quo must be preserved at any cost.
So it was that God called the Gentiles, both the good and the bad. The truth is, being "good" is no guaranteed passport to heaven. The Pharisees were "good," after all, following the Jewish Law to the letter. Merely being "good" is never enough. Salvation is granted unto those who strive with violence to be made worthly (by God's grace) of the Kingdom.
So the magnificant feast commences, but lo and behold, the king enters the banquet hall and finds there a guest who is not wearing a wedding garment. He is therefore unworthy: but why? Because in those day, the guests were not expected to bring their own garments. These were provided at the door to all who entered. Therefore, this guest is unworthy because he rejected that which was so graciously provided, free of charge. That is, he rejected the grace of God, which alone can make us worthy of the Kingdom.
This parable was spoken to the Jews, but just as surely it can be applied to so-called Christians of this day and age. Many there are who make light of their salvation, being totally focussed on worldly goals. Others do not even bother striving to acquire the grace of God (which is, according to St. Seraphim of Sarov, the whole purpose of the Christian life) because they feel they are already "good enough."
So it is that "few are chosen:" not because God does not desire our salvation, but because we ourselves reject this gracious gift either through indifference or laziness. It is we ourselves who choose either life or death, but this is not a one time choice. Rather, we make many choices, every day of our lives. Every time to choose to put anything whatsoever before our commitment to God, or we place worldly comforts and pleasures before concern for our salvation, we are choosing death over life.
It is, in fact, the sum total of our choices throughout our earthly life that determines our eternal destiny, and the more often we choose death over life, the more difficult it is to break the pattern. However, that which is impossible to man is possible to God. No matter how far we may have gone astray, it is always possible through a single decisive choice combined with sincere, heartfelt repentance to be restored to the path of salvation.
The most vital thing to remember, however, is that today (this very moment) is the day of salvation. Tomorrow it may be too late, and the doors to the wedding feast may be closed forever.