Theonomy is the belief that God's Law (which includes the Mosaic Civil Law) has an "abiding validity" in "exhaustive detail." With the impression given of a total reenactment of the Mosaic law-code in today's society, it is not surprising that there is a lot of resistance to the idea, conjuring up images of the Inquisition for example. Now, whether theonomy actually leads to the notion of holy wars is an interesting discussion to have, but for now I would like to look at an argument its foremost proponent, Greg Bahnsen, offered against a critique of theonomy.
In his book Theonomy in Christian Ethics, Bahnsen attempts to put forward his case for theonomy. In dealing with the Sermon on the Mount, Bahnsen notes the argument is offered (against theonomy) that Jesus did away with the punishment of adultery as "prescribed in the Old Testament" (p. 111), thus theonomy, which states that the Mosaic Civil Law continues on in "exhaustive detail," is false. In response, Bahnsen replied that Matthew 5:32 deals with "divorce and its proper ground, not adultery and its proper punishment" (p. 111. Emhasis original). In other word, Bahnsen asserts that nothing is said here in Matthew 5:32 about the punishment for adultery, and therefore one cannot claim hat Jesus did not hold to the death penalty prescribed for adultery in the OT.
Now, on the surface this sounds true. Surely Jesus was focusing on the sin of divorce, and the part about adultery, or rather the broader category of sexual immorality, is the exception clause to his main point. Yet, if we put more thought into it, the argument doesn't seem to hold up.
The OT punishment for adultery is death, with the exception of consensual sleeping with a virgin which then leads to forced marriage (Ex. 22:16). Once the adulterer is stoned to death, the remaining spouse by definition becomes a widow/ widower, and is thus able to remarry. Therefore, if Jesus held to the "abiding validity of the Law in exhaustive detail," then the exception clause becomes just another way to describe the normative situation when the adulterer is dead. How is that an exception clause at all, since everyone acknowledges that remarriage after one's spouse is dead is permissible?
Deuteronomy 24:1-4 deals with divorce because a man found "some indecency" in his wife. Whatever the "indecency" refers to, it cannot be adultery since adulterers are to be stoned, not just divorced. Putting the Deuteronomy passage besides Jesus' teaching on divorce suggest dissimilarities between them. The most that can be suggested is that the "indecent thing" (עֶרְוַ֣ת דָּבָ֔ר ; LXX ἄσχημον) is a broader category of which sexual immorality (πορνεια) is a subset, for the one divorced in Deuteronomy 24:1-4 is not an adulterer. The Mosaic code therefore suggests that divorce is permissible for other things than adultery, which is clearly not what Jesus taught in Matthew 5:31-2. Regardless of how one reads the exception clause, one thing that we can agree is that the exception clause gives at most one basis for divorce, that of adultery or sexual immorality. Finding an "indecent thing" does not seem to be an acceptable reason for divorce according to Jesus, whereby it was under the Mosaic code.
So yes, Bahnsen is right that the text deals with "divorce and its proper grounds," not "adultery and its proper punishment." Yet, we have shown that (1) the Mosaic grounds is broader than Jesus' grounds, and (2) the exception clause makes no sense if we assume the penal sanctions of the Mosaic code. A text does not have the topic of importance as its main theme in order to say something about the topic, and we have seen how Jesus' words actually shows dissimilarity with the Mosaic code on the issues of divorce and adultery.
Bahnsen's main thesis is that God's Law, as seen especially in the Mosaic code, has abiding validity in exhaustive detail." It is however seen here that at least on this topic, the details do not have abiding validity, and therefore theonomy is wrong at least with respects to this particular topic.