It seems to me that the Theonomist argument can be boiled down to the following Syllogism:
P1) God does not change.
P2) If the Law has been abrogated, then God has changed.
C) The Law has not been abrogated.
The Reformed Libertarian objection is with P2. This is not necessarily true. If the purpose for giving the law in the first place was so that it would be a typological and eschatological foreshadow of things to come with the intention all along that it would be abrogated when those things came to fulfillment, then it is not a matter of God changing for him to abrogate the law now that those things have come to be. Rather it is a symptom of God NOT changing that he would be faithful to fulfill that plan.
For example, when Israel committed the sin of the golden calf at Mount Sinai and God was going to wipe them out, Moses interceded for them. He reminded God of his promises, and God appears to change his mind. It almost looks like God is a nearly unhinged human person who Moses talks sense to until he calms down. It even uses a word that is very similar to “repent” for what God does here.
So did God change? Certainly his course of action did! Does this mean he is no longer immutable? No longer simple?
Of course not. It meant that his purpose all along was to evoke this intercessory ministry from Moses. He never really intended to destroy Israel because he knew and sovereignly ordained what would happen. One major reason for this was because it gives us an amazing picture of Jesus’ intercessory work for us.
The same idea or pattern holds here. The judicial/civil Mosaic Law for Israel was meant, in large part, to foreshadow the coming greater Kingdom of Christ. That Kingdom is now here in a different sense, and that sense clearly does not involve using the sword against outsiders (1 Corinthians 5).
So the theonomist argument is not proven by this syllogism. Rather it begs the question of whether God’s purpose was for the law to be a type/foreshadow or whether it was intended to be set in stone for all ages.
And the answer to that question will inform our interpretation of what Jesus means by what he says in Matthew 5:17-18.
“Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished.”
What is meant here by these words “abolish”, “fulfill” and “accomplish”? What we understand the original purpose of the Law to be will inform the definition of those terms, won’t it? It will lead us to a certain view about whether and to what extent Christ’s life, death and resurrection fulfilled and accomplished some or all of it.
The Reformed Libertarian position, from considering the whole of Scripture and especially the whole of the New Testament, is that yes the Law is typological. It has indeed NOT been abolished, but has been, in large part, fulfilled and accomplished by Christ and the New Covenant which it was foreshadowing. There are nuances to this view, but the overall point is the same: The New Covenant is the Covenant of Grace, the Old Covenant (including Abraham’s) were all Covenants of Works with Grace coming retroactively from the New as a foreshadow. All the preceding Covenants were typological of the New and all must be interpreted in light of that typology, including elements like the Law of Moses.
No the Law has not been abolished. Certainly the moral principles of the Law (the standard of God’s holiness it sets forth for all people and especially his covenant people) is very much the same today as it was then, and the Law is inestimably valuable at teaching it to us, for it is God’s holy, infallible, inerrant, and sufficient revelation of it.
Yet it’s application to the New Covenant must take into account the ways in which Christ has fulfilled and accomplished the typology of the way it was applied in the Old Covenant. Therefore the practical application will of necessity look different today (1 Corinthians 5 gives us a hint of this).
And whatever else we believe about baptism or sabbath keeping or the second commandment or whatever…. One thing is crystal clear to us Reformed Libertarians: the standards of the Law are no longer to be enforced in exhaustive detail with the sword by the civil magistrate. For God’s covenant community no longer takes the form of a civil nation. Instead, the purpose of the sword bearing magistrate is to defend the life, person and property of those who “do good” (Romans 13).
Truth & Liberty 
This is one of the few references on the Web to typological fulfilment of the civil law of moses. As far as I can tell the traditional approach has limited typology to the ceremonial law and not the civil law.
It’s a fertile and interesting approach to study how and when the civil law of moses was fulfilled and what the law of moses fulfilled looks like in terms of the new covenant civil law. Have you taken this approach further?
Mike, I appreciate the biblical aspect of your argument above. The first half of this reads like a New Covenant Theology argument. 😉
Which is the reason for my comment. You (along with all of Christendom) assume the ceremonial dimension of Mosaic Law as fulfilled by Christ and, thus, abrogated. Here, you assume the civil dimension of Mosaic Law as fulfilled by Christ and inauguration of His Kingdom and, thus, is abrogated as a law to us. Yet, you (and many Christians over the last 500 years) feel comfortable not seeing Christ as fulfilling the moral dimension of the law, and, thus, abrogating that as our law.
If Jesus is not Himself the exact and perfect representation of the true Holiness of God, and thus our Standard, then why not? Surely, Jesus filled all 3 dimensions of Mosaic Law to their fullest representation. And Jesus’ righteousness and holiness + our (lack of) righteousness still equals Jesus’ righteousness and holiness. We do not live out any dimension of Mosaic Law because we have Jesus’ imputed righteousness to simply embrace, which is the fulfillment of the whole of that law-covenant.
True, there is such a thing as right living, but we need not look past Christ to Moses as though his 10 words are our life. Their fulfillment by Christ is our life, and we ought to live by that Life, and not by the lesser. Christ is greater than Moses. I think we must read Matt. 5:17-18 in light of both II Cor. 3 and Hebrews 8-9 to get a full picture of what Matt. 5 is saying. To your point, God has not “changed” even though the Old Covenant has been fulfilled in the New Covenant in Jesus’ blood; it was how it was supposed to be from eternity past.
I think we can get into trouble when we read the New Testament as though its fulfillment and abrogation of Mosaic Law excludes the moral dimension. The whole is fulfilled, and abrogated. And I need the imputed “moral-ness” or righteousness of Christ just as much as I need the imputed atonement, purification and consequent reign of Christ over me in His Kingdom. While there is substantial overlap between eternal morality and Mosaic morality, we should not assume it is one in the same.