Let's say, heaven forbid, you're having a conversation with someone from the Exclusionist camp of Messianic Judaism (e.g. Derek Leman, Boaz Michael, etc). You want to give a One Law rationale for kashrut law and so you say something like the following:
"But, Derek, kosher animals are qualitatively different from unkosher animals. Unkosher animals are bottom-feeders, scavengers, the nutritional value of the meat is poor, there are health risks, etc. In other words, G-d doesn't want anyone to eat garbage. Therefore, kashrut law is for everyone!"
He would then come back with something which would destroy your argument:
"Then why does Torah say that un-covenanted Gentiles could eat treif meat?"
Whoops! He'd have you there!
The Bible does INDEED say that kashrut law is only for those who are IN COVENANT with G-d.
See where I'm going with this?
In other words, base your argument on the fact that there is only one covenant with the people of G-d (Israel). The New Covenant is a renewal of the Sinaitic Covenant. The New Covenant contains the same Torah as the Sinaitic Covenant--because it IS the Sinaitic Covenant!
Let's look at an actual example of something Derek wrote (lest anyone accuse me of putting words in his mouth):
"...God permitted Noah to eat all living things. It’s there in Genesis 9. It was never repealed. God never enforced the dietary laws of Leviticus 11 on gentiles. Noah was allowed to eat pork and shrimp. Israelites were not. We’re not all the same. Oh sure, people who realize this about Noah, but who don’t want to believe it, will perform scriptural gymnastics to deny what is plain. But not only are their arguments about “clean” and “unclean” animals in Noah’s time simplistic, they also cannot explain Deuteronomy 14:21." (from derekleman.com. located here: http://www.derekleman.com/musings/were-not-all-the-same/)