At this point in Mark’s gospel, Jesus is “on the way” to Jerusalem with his disciples. He’s been teaching them about what it means to follow him. The recurring theme is discipleship as no small matter. It involves a whole new way of thinking about such concepts as greatness (9:33-36), judgment (9:42-50), and, now, marriage. 
This text begins with a test from some Pharisees, but neither Jesus nor Mark focus on it (For background on the differences between Shammai and Hillel on acceptable grounds for divorce, see any substantive commentary). Instead of taking sides in the debate, Jesus takes the opportunity to teach about the nature of marriage. What does he say? Marriage is an intimate and permanent union between man and wife. It has been that way from the “beginning of creation” (ἀρχῆς κτίσεως). Moses allowed for divorce, but not because it was okay with God under certain circumstances. It was because of Israel’s stubbornness, obstinacy, and “hardness of heart” (σκληροκαρδίαν). Those who seek to justify divorce, for whatever reason, miss the point. Jesus’ conclusion is clear and simple: “What therefore God has joined together, let man not separate.”
Preaching on this text presents several opportunities. First, it is an opportunity to talk about marriage and divorce. When the church today speaks about marriage, it tends to focus on the definition of marriage and the “marriage equality” debate, but that is not where Jesus focuses in this text. Second, this text is an opportunity to consider a deeper problem we all face which has to do with faithfulness.
A sermon on this text could address both issues.
Statistics are always questionable, but it appears that divorce rates inside the church are similar to those outside. If that is true, one reason may be the unhelpful concept known as “biblical grounds for divorce” (Google it and notice how many Christian sites sound like Pharisees looking for loopholes). In the church we often approach divorce backwards. We look the other way before divorce takes place, but after the divorce has occurred, we treat divorced brothers and sisters as second-class Christians. This should not be so. Pastors and congregations should do everything in their power to prevent divorce from happening among Christians in the first place. This includes more teaching about the permanence of marriage as well as offering help (from the pulpit even) toward reconciliation for couples going through hard times. And when divorce has happened? Regardless of the cause, when divorced individuals return to the Lord in repentance, they should be treated as fully forgiven and restored members of the body. This will include offering tangible and sustained support as spouses (and children) deal with the fallout that inevitably comes with a broken home.
*It should be said (here and in a sermon on this text) that there are times when divorce may be the lessor evil of two terrible options, but such circumstances are probably rarer than we think. When they do arise, we should be clear that, even then, divorce is never God’s will.
No matter which angle you approach the text from, God’s will for (or against, I should say) divorce cannot be the focus of the sermon. The preacher is called to proclaim the Gospel, the promises of God in Christ. This is where the second issue comes into the picture. The presenting question about divorce in the text exposes a much deeper problem we all face; whatever our marital status. It is our lack of faithfulness. We live in a world that constantly looks for loopholes, exceptions and excuses to justify unfaithfulness—whether in marriages, business dealings, friendships, or social commitments. Christians do this too. This is a problem, for God calls his people to imitate his other-worldly faithfulness.
Which is where the sermon must ultimately lead. God’s faithfulness to his people is nothing short of astounding. The prophets and apostles often used the marriage metaphor to make this point. With it they called their hearers to repentance (Jeremiah 3:1-14; 16:1ff; Hosea 2:1-13; 4:1-3; 2 Corinthians 11:2) and proclaimed God’s long-suffering faithfulness to his people, his bride, his church (Isaiah 54:5; Hosea 2:14-23; Ephesians 5:22-23; Revelation 21:2). This divine faithfulness is seen most clearly and definitively in Jesus, the faithful Son in whom we have eternal fellowship with God (1 Corinthians 1:4-9; Hebrews 3:1-6). His faithfulness to the Father for our sake