Thus, chapter 12 is important because it shows what the "temperature" of Jerusalem was like in this last week. Notably, the chapter details confrontations between Jesus and the religious leaders. The "chapter" really begins with the confrontation at the end of chapter 11, which follows the barrenness of the fig tree. As Jesus and the disciples re-enter Jerusalem, he is met by chief priests, scribes and elders who question his authority. Their refusal to answer Jesus' question in reply reveals that they are fearful of the people.
Jesus then tells a parable about how he will be rejected, as the son, by those who have merely leased a vineyard but desire to take it over. This puts the true motives of the leaders in the forefront. The vineyard is the stewardship of the people of God, but the leaders, over history, have rejected the owner's attempts to manage it. They finally kill the son in an attempt to become the heirs. The conclusion is that the they will be tossed out by the owner when he returns. This conclusion, Mark reveals, results in the leaders deciding to lay hands on Jesus.
Jesus then has negative confrontations with the Pharisees and Herodians and the Sadducees. The first interaction has to do with a civil matter, the second with a "biblical" matter. In both cases Jesus demonstrates how the questioners are in error. In Jerusalem, Jesus is confronted by the chief priest, scribes, elders, Pharisees, Herodians and Sadducees. The entire gamut of spiritual leadership confronts him. Each challenge is a challenge over the integrity of his authority.
Finally, a scribe approaches Jesus to discover the truth. His question also has to do with the authority of Jesus. He wants to know what the "first commandment" is. Jesus answered him, that the "first commandment" is to love God and to love your neighbor as yourself. The man concurs with this answer, and Jesus tells him he is not far from the kingdom.
Finally, Jesus teaches them about his identity in a way that traps them. They think that the Messiah who was to come (but who was really among them already), the Son of David, would be a human figure. But Jesus teaches them that he is divine. He asks how the scribes can call the Messiah the Son of David when David himself describes him as Lord.
This chapter closes with Jesus comparing the scribes, who devour widows' houses for their own purposes, with an actual widow, who surpasses the righteousness of the scribes, when she gives everything she has to the temple treasury to support God's houses.
This chapter has contained a lot. The entire spiritual leadership of Jerusalem confronted Jesus about his authority and identity. But Jesus showed his true authority by not only rebutting them, but by showing that he is the true interpreter of what the law really means. And he established his identity as the Divine Son of David, the Lord (aka, the Messiah).
Jesus also shows, in this chapter, how to follow him. To not be barren (literally, like the fig tree; figuratively, like the religious leaders), a follower of Jesus must focus on the "first commandment" -- must love God completely and must love neighbor as self -- and must be devoted to giving to God. Jesus does not mean that we must physically give as much as we can to the "temple." Rather, he exemplifies the attitude of the widow, who dedicated her entire livelihood to the work of God.
There is much here to comment on, but perhaps this suffices: Are you more concerned with protecting what you have and running God's "program" through your concerns; or are you giving everything to God, and sacrificially and selflessly loving him and your neighbors?