The contrast could not be more striking. On the one hand are the scribes; the experts in the law, the scholars, members of the religious elite. With long robes (στολαῖς) and even longer prayers (see Matt. 6:5-6), they made their presence known wherever they went. These were men who mattered. Then there was the widow. Unlike his detailed description of the scribes, Mark says almost nothing about the widow. He mentions only her poverty. She appears nameless and (to everyone but Jesus) useless. In the end, she was entirely penniless. 
Both the scribes and the widow were in the temple that day. They were close in proximity, but, in relation to the kingdom of God, they could not have been farther apart. Jesus himself highlights the contrast, which invites us to pay close attention.
The contrast is between those who work hard to show their importance and those who give of themselves without pretense. The scribes suffered from “self-intoxication” (France, 489). They stand as a strict warning to contemporary readers of this text: do not think of yourself more highly than you ought (Rom. 12:3; see also Phil. 2:3-4). This is not new in Mark. It is a theme that has been building for several chapters. Jesus has been lifting up the least, the last, the child, and the foreigner. He has been demoting those who put themselves forward, whether it be the scribes in this text or the disciples who asked to sit at his right and left (10:35-45). This is how things go in the upside-down kingdom of God. Jesus lifts up the humble and humbles those who lift themselves up.
What might the preacher do with this contrast?
We could start with what not to do. Making too close a connection between this woman and the widow of Zarephath in the Old Testament reading would be a stretch. While they both live by faith in various ways, the multiple and significant differences between the two narratives suggest the preacher should choose one text or the other. Likewise, a sermon on this text that simply tells the hearers to be more like the poor widow misses the point. She did not present herself as a model. Neither did Jesus tell his disciples to follow her lead.
Instead, the preacher could use this text to call out all manner of pretension and self-promotion among the hearers. This is a real temptation for regular church goers (and preachers!). Faithful Christians routinely, if indirectly and unintentionally, find themselves impressed with their own faithful Christian living to the neglect of many nameless “widows.” They should heed Jesus’ warning. Beware!
This still is not the gospel, however. To make sure the sermon proclaims the good news, the preacher could continue a Markan theme that highlights the upside-down nature of the reign of God in Christ. Jesus reigns by way of opposites. Philippians 2:5-9 is a great example of this. So is the epistle reading for this week from Hebrews 9:24-28. Jesus gave up his seat of honor in sacrificial and humble obedience to the Father. In this light, the widow in the text is a type of Christ. Like her, Jesus allowed himself to be devoured by the religious elites, giving everything for the sake of his people. The sermon, then, would use this text to proclaim the self-emptying of Jesus for us and for all people.
As hearers live and believe in Jesus, their lives will grow more and more to resemble the lavish giving of the widow. The preacher could point in that direction, but the focus of the sermon would be on the selfless and sacrificial work of Jesus for us.
Lectionary Podcast: Dr. David Scaer of Concordia Theological Seminary, Ft. Wayne, IN, works through Mark 12:38-44.
Lectionary at Lunch: Multiple resources for preaching Mark 12:38-44.