Forgive me if this is an elementary question...
Do the Blessings and Curses in Deut 28 apply to all believers NT & OT, or only ethnic OT Israel?
It does apply to us. The only thing that changed from the Old Testament to the New Testament are the cermeonial laws and the sacrificial system which were all fulfilled in Christ. The ethical requirements, and stipulations with their relative penalties still remain in force.
Proof.
All men and nations implicitly fall under the covenant since they descended from Adam. This is so whether they know about it or not or whether they acknowledge it or not.
1.God destroyed the antediluvian world with a flood because of their wickedness.
2. God judged Sodom and Gomorrah because of their abominations.
3. God judged Egypt with plagues because of Pharaoh's and the nations disobedience.
4. God judged the Canaanites, Amorites, Amalekites and the Hittites because of their abominations.
In none of the above cases did God explicitly made a covenant with them, but it was understood that the broke the laws of some covenant. Israel got the Covenant and the laws explicitly codified to them because God was
gracious to them. He didn't allow them to wander in ignorance and to be cut off like the heathens.
5. God sent Jonah to Nineveh to urge them to repent. If Jonah didn't go they still would have been judged.
6. God judged Babylon because of their wickedness.
7. God judged the Persians and the Greeks.
8. God judged the Israelites again in A.D 70.
9. Then He went on to judge the Romans.
10. And wicked nations continue to be judged to this day. Just look around.
11. For a closer look, look at wall street where whole companies have been dissolved because of greedy executives. Look at the greedy homeowners who have their houses foreclosed on. Look at the woman who committed abortion that ends up in the psychiatric ward and driven to suicide, look at the homosexual that contracts AIDS, look at the career woman that reached her 40's and is rich, childless and miserable. Look at whole nations in Africa that were committed to communism now totally bankrupt, impoverished and corrupt. Look at the middle east as the wallow in the idolatry of Islam which manifests itself in suicide bombings, honor killings of their own daughters and economic stagnation. Look at China, the godless statist country that imposed 1 child per couple policy leading them to abort subsequent children. Look at how they constantly suffer the blight of earthquakes, and floodings which claim the lives of 1000's, not 100's or scores as the U.S tragedies.
Jesus came to fulfill the law not to abrogate it. His fulfillment came in keeping the law and offering the perfect sacrifice so that the breakers of the law who repent can be justified. To say that God's revealed law and its stipulations and sanctions are no longer binding to individuals and nations is to say that Christ came only to give individuals and nations a license to sin.