This segment of Scripture goes from good to bad, in just about 30 verses. We start with the positive, cheerful idea that those who support and encourage a ministry or a follower of Jesus are rewarded just as those who are encouraged and keep on in their good works. Then we finish with the idea that judgment day will be better for Sodom and Sidon than for the cities that rejected Jesus...
That's a mood shift in your daily Bible reading for you. Let's take the pieces and put it together:
First, there is a unifying theme in these segments: the work of spreading the message of the Gospel. The first section deals with those who encourage the messengers, the second with the discouraged messenger, and the last with those who ignore the proclaimed message. The thread of messages and messengers tracks across this set of passages.
So, what about it? First, to recognize that some people are charged with spreading the message--the opening section deals with prophets, righteous people, and little ones. In this context, each of these terms deal with those fulfilling a responsibility in a religious context. (Other places, "righteous people" is a general term and "little ones" is more about children/younger folks.) Here, though, you should these as people serving God. Some are well-known, easy to see, like prophets. Others have distinguished lives, like righteous people. Others are simply those walking humbly through life--the little ones. (Mostly derived from: Morris, Leon. The Gospel according to Matthew. The Pillar New Testament Commentary. Grand Rapids, MI; Leicester, England: W.B. Eerdmans; Inter-Varsity Press, 1992.)
Jesus is pointing to the idea that these folks cannot fulfill their tasks without support, and the supporters are equal partners in the work. A side note would be the counter-condemnation that the supporters of those who do evil, down to simply offering a cup of cold water to aid those doing evil, should expect the same reward as the ones doing evil.
Then we see the twelve disciples sent out to preach, but Jesus does not leave their hometowns untouched. Instead, He goes and preaches there. This is a direct pushback against the mindset that we do not need to do missions because there are plenty of needs here: both need to be done, and God is sovereign and capable of meeting the needs here if we go there.
John the Baptist gives us an interlude, as he struggles with his time in prison. He's not entirely sure Jesus is who John thought He was, but then Jesus uses that to highlight the work of John.
Finally, Jesus points out the condemnation due those who reject the message. He highlights the amount of mercy shown to the cities He has visited and how other places would have responded with repentance had they seen what the current cities experienced. That raises this question: of all the blessings we have received, have we met the Lord God in repentance for our sins?
Or do we hold on those sins? What judgment will we face for embracing the very sins God has condemned?