Setting and Overview
Divorce was a serious issue in Jesus’ time, and ironically enough, it was especially prevalent among the Pharisees. Divorce is a serious and destructive event, so preventing unnecessary divorces would seem to be serious enough itself to warrant attention, but the dissolution of marriages is not the only issue. The decline of marriages was both a reflection of the deterioration of the marriage relationships and also that the Jewish people had lost sight of the ideal of the marriage relationship and what it was supposed to represent. In calling the people to no longer minimize divorce, He is also pointing them back to how God’s people are supposed to be devoted to Him.

Exposition
Jesus begins verse 31 by quoting Deuteronomy 24:1. This was a well-known verse and one that is referenced in the Mishnah’s teaching on divorce. Deuteronomy 24:1 refers to a man giving his wife a certificate of divorce, reflecting the fact that by that time, divorce was not uncommon (Leviticus 21:7,14, Deuteronomy 22:19, 29.) This was not a justification for divorce (Malachi 2:10-16), but rather a source of protection for the wife if the husband does divorce her. The focus of Deuteronomy 24:1-4 is actually to condemn remarriage to that wife if she has in the meantime had another marriage (Jeremiah 3:1.) Different rabbis at the time, however, focused simply on the first verse and used it to justify marriage for various offenses, ranging from sexual sin by the wife (so Shammai) to the wife burning dinner (so Hillel.) Their focus in both teaching and example, was simply that a certificate of divorce should be given, and if so, then then nothing wrong was being done.
Against such a mindset, Jesus gives His command. Except for situations of “immorality,” divorce is not permitted. In fact, He says that to do so is to cause their wife to herself commit adultery. The reason is that if there is not proper grounds for the divorce, then they are still bound by the first marriage, and if they then get remarried (which they would have little choice but to do if they wanted a means to provide for themselves), then they are committing adultery on their first husband. Jesus will deal more with the reason that divorce was permitted under Mosaic Law in Matthew 19, but for now the focus is to remove the illegitimate reasons used to justify divorce. There is still some debate over what all would be included in “immorality,” but the most common understanding is that it is a reference to a wife who has committed adultery. Outside of the specific situations where divorce is permitted for Christians (see also I Corinthians 7), Christians should not see it as a viable path to consider.

Examination and Application
For those who are in a situation where they feel that divorce may be the best, or possibly, only solution, then this passage does have a very immediate and relevant context. Christians who are in that situation or who know someone who is should consider this passage carefully, along with Matthew 19, Mark 10, Luke 16, and I Corinthians 7. However, the application of this passage extends far beyond that. In pointing out the horror of divorce, Jesus is pointing us back to what marriage is supposed to be and what it is supposed to represent. As we follow the arc of Scripture from Genesis 2:18-24, to Exodus 19:3-5, Isaiah 54:4-5, Jeremiah 2:1-3, 5:7, Hosea 2:14-20, John 3:28-29, 14:2-3, II Corinthians 11:1-3, Ephesians 5:25-27, 29-32, and finally to Revelation 19:6-9 (as just one path among many that we could take) we see a picture of how God calls His people into relationship with Himself, and despite our failure to be the bride that He has called us to be, how He reconciles us to Himself to present us to Himself as a worthy bride, and we look forward to the full realization of that relationship as we are brought into perfect, eternal, relationship with Him. In the meantime, He calls us to strive to having marriages that remind us of that promise, and reflect to the world His perfect love for us.