"17 And [Jesus] came down with [the twelve] and stood on a stretch of level ground [with] a great crowd of his disciples and a large number of the people from all Judea and Jerusalem and the coastal region of Tyre and Sidon 18 came to hear him and to be healed of their diseases; and even those who were tormented by unclean spirits were cured. 19 Everyone in the crowd sought to touch him because power came forth from him and healed them all.
20 And raising his eyes toward his disciples he said:
"Blessed are you who are poor,
for the Kingdom of God is yours.
21 'Blessed are you who are now hungry,
for you will be satisfied.
Blessed are you who are now weeping,
for you will laugh.
22 'Blessed are you when people hate you
and when they exclude and insult you,
and denounce your name as evil
on account of the Son of Man.
23 'Rejoice and leap for joy on that day!
Behold, your reward
will be great in heaven. For their
Ancestors treated the prophets
in the same way.
24 'But woe to you who are rich,
for you have received your consolation.
25 'But woe to you who are filled now,
for you will be hungry.
Woe to you who laugh now,
for you will grieve and weep
26 'Woe to you when all speak well of you,
for their ancestors treated the false
prophets in this way."
First of all, let me point out certain differences between the Sermon on the Plain vis-à-vis the Sermon on the Mount for basically they are the same Beatitudes that Matthew wrote in Matt.5: 1-12. The difference of course is that the Sermon on the Mount only talks about the blessed things people can have, while Luke's Sermon on the Plain gives not only the blessings, but also the "woes" or curses if you do not follow the teachings of God.
As the four evangelists only began writing about Jesus long after he ascended into heaven, many of them may have been confused about the events that happened. There is no telling if Jesus gave us the beatitudes or the Sermon on the Mount together with the Sermon on the Plain? I have not been to the Holy Land, but those who have been there tell me that where this happened seemed to have both a huge hill and a plain down below.
As you've known by now, St. Luke was a second or a third generation Christian and probably read certain manuscripts about the beatitudes and sought certain Jews who were eyewitnesses to these events. What they probably got were conflicting reports about this having been said these on a mountain or on a plain. But let's not get ourselves confused on where the message was said. Rather we should focus on the message itself.
No doubt, the Beatitudes are self-explanatory, so we'll just use some basic examples. When Jesus said, "Blessed are you who are poor for the kingdom of God is yours." I'm sure that Jesus was referring to those who are also poor in spirit, not just the materially poor.
If you ask me, the "woes" that Jesus teaches us is so logical and it should be taken as a warning to all. For instance when he said, "But woe to you who are rich, for you have received your consolation. But woe to you who are filled now, for you will be hungry." This passage isn't a condemnation of those who are rich in material things. Rather it is a reminder that someday if you do not use your wealth to help others, you too will be hungry.
Jesus always talks about spiritual matters, so when he warns us that someday you too will be hungry, he most probably meant that you will be hungry for the presence of God. The Sermon on the Plain is different from the Ten Commandments or Decalogue that gives us "Thou shalt nots". Giving us "woe to you's" is more of a soft warning than a stern... don't do it! What we need to accept is the Love God has for each one of us and then we would never do anything to offend someone who loves us so much.