After he makes a clear statement that divorce is not permissible for any reason, the Pharisees sense they have an advantage over Jesus. Even if marriage is part of the created order, Moses mad an exception and permitted divorce in some cases. Jesus says this exception was granted “because of your hard hearts” (v. 8). Jesus does not quote scripture here but explains why Moses included that exception.
The reason for the exception is humans have “hard hearts.” The noun σκληροκαρδία and related terms are common in the Greek Old Testament (Deut 10:16; Jer 4:4; Prov 17:20; Ezek 3:7), usually related to stubbornness of Israel to keep the covenant. It refers to an “an unyielding frame of mind” (BDAG), a mindset that can only be resolved by “circumcising your heart” (Deut 10:17; Jer 4:4). Ezekiel 3:7, “all the house of Israel have a hard forehead and a stubborn heart.”
A spouse may divorce an unfaithful spouse (not just men). Unfaithfulness (πορνεία) is a word which covers a multitude of sins. The noun refers to fornication in general, from the specific act of visiting a prostitute to any sexual union outside of marriage. Robert Gundry says there is “no need to adopt obscure definitions of πορνείας, such as marriage within the forbidden degrees. The specific word for adultery does not appear in the exception phrase simply because a general expression occurs in Deuteronomy” (commenting on the Sermon on the Mount, Gundry, Matthew, 91).
Although the word specifically refers to sexual sin, perhaps “unfaithfulness” is better translation. The “exception” cannot refer only to adultery since a spouse can harm a wife and children in other ways, they are unfaithful to their role as a husband, spiritual leader, etc. This teaching does not say to a wife who is regularly beaten by her husband, or her children are abused by an evil husband, “you can’t get a divorce, just try to work it out.”
This is an important place to pause and ask if the Mosaic Law is applicable to the church. Paul is clear the Body of Christ is not under the Law, so can’t we ignore all this discussion about how Deuteronomy should be interpreted? In 1 Corinthians 7:10-16, Paul seems to allow divorce in other circumstances (an unbelieving spouse leaving a believing spouse). Does Paul create “wiggle room” for divorce in 1 Corinthians 7:12-16?
Jesus then declares marriage after divorce is adultery (v. 9). This is a very difficult verse in terms modern practice. There are more reasons to get out of a marriage than sexual unfaithfulness. Many modern Christians extend this unfaithfulness to “the wedding vows” in general, so that if a husband is abusive and harms his wife and children, she should get out of the marriage for her own protection. Sometimes divorce is necessary because of medical debt, a couple might divorce to keep one spouse’s medical debt from bankrupting the family! Some older couples are better off living together to not lose social security benefits. These are things Jesus is not talking about in this passage.
On the other hand, contemporary culture has similar easy and legal divorce for any cause so that someone might respond to typical marital difficulties simply by walking away and maybe starting over with someone else. Jesus’s answer would be, understand the spiritual union of marriage in the first place, understand the level of commitment you are making to another person.
An important observation: Jesus is talking to his own followers here, not making a general statement for the entire world. While the created order does favor marriage as a mystical union intended for a lifetime, the church cannot impose that teaching on people who are not Christians!
Jesus is clear: adultery is sin and divorce is not God’s intention for marriage. But there are many sins which the church does not seem to worry as much about, gossip, greed, coveting, etc. There is nothing in the Bible implying a divorce result in a sin that is “more damning” than being a gossip.
The point of this section is less on divorce than on the importance of a right understanding of marriage in the first place. If a couple understands what they are doing when they marry, that God is joining them together as “one flesh,” then the less likely the need for divorce. “Christian ethics in a fallen world will always be subject to such tensions; Sinful situations sometimes make it impossible to implement the ideal, and in such cases we may have to choose between courses none of which leaves room for regret” (France, Matthew, 282).