Greg responded by saying that the challenge is based on a misunderstanding of what Deuteronomy is:
The Mosaic Covenant is an agreement that God made with Israel; it is not an agreement God made with anyone else.... Nothing that is in the Mosaic Law applies to me as a Gentile in virtue of it being in the Mosaic Law. No command is incumbent upon me in virtue of being part of the Law.
Here’s the best way to characterize this: You and I both live in California. There is nothing in California law that applies to somebody living in Ohio. Ohio has its own statutes. Now, there is a statute against murder in California. Does that mean since I’m not in California, it’s okay for me to murder in Ohio? No, I’m not under the California statute, but I am under the Ohio statute.
And part of the reason is that there are a lot of rules that are in statutes that turn out to be the kind of rules that should apply in any situation in any state, and you’re going to find them repeated. But then there are also going to be laws and rules that are unique to that state’s situation.
Think of the Mosaic Covenant like a state Law. It was given to the people in that state for that period of time. We are in a different state now. We have a different covenant that applies to us (and that’s the New Covenant) and different moral obligations. We are not obliged by everything in the Mosaic Law. That was a set of civil obligations that were applicable to those people under that time. It was made between God, and Moses, and the Jews....
Does that mean we can go around murdering people because we’re not under the Mosaic Law? No, the obligation not to murder is a universal and should be in any Law....
[A person might say,] “Well, you’re just picking and choosing.” No, I’m not; I have a rationale why this doesn’t apply in the same way and why others might. And the things that do apply as universal moral obligations from the Mosaic Law are going to be repeated in the New Testament to Gentiles. These things are not repeated, because we are not under that theocracy. It’s just exactly like being in a separate state.
To hear Greg’s full answer, listen here (the question starts at 00:36). For a more lengthy discussion on the relationship between Christians and the Mosaic Law, see 40 Questions About Christians and Biblical Law by Thomas Schreiner.