Second Chance's Reply # 2 is well stated and more explicit than the supplement I am offering here. Ancient Macedonia was a City-State of Greece, just as Sparta and Athens were, essentially self-governing provinces. The territory which is considered Macedonia today, is the result of border changes during the Balkan Wars, before, during and immediately after WW I, and is within Serbia, Bulgaria and Greece. Even in ancient times, Northern Macedonian's were Slavic people, in the South they were Greeks. The nation which claims the name Macedonia today, is a tiny area, perhaps comparable to a large urban and suburban area, with rural outskirts. It by no means encompasses the ancient province of Macedonia, and that is one reason why today's Greece refuses to permit this tiny area around Macedonia's capitol city to claim the name Macedonia. Greece has a Northern province named Macedonia.
When the borders of the Greece, Serbia, and Bulgaria were settled after WW I, the respective Orthodox Churches agreed to correlating territorial boundary changes.
The so called "Macedonian Orthodox Church," was promoted by Yugoslavia's Communist Dictator, Josef Tito's administration, in his ongoing effort to diffuse the strength of Serbia and its church. It is not recognized by any of the Holy Orthodox Churches.
If you don't mind, I would like to offer a few comments on your supplement. Regarding the idea that a nation (as we now define it) called Greece existed in antiquity is not supported by historical evidence. You are correct that what we had were city-states, who placed their own interests above that of (to them) a mythical nation. There is no doubt that they considered Greeks to be different and better than non-Greeks (for example, Macedon was initially called barbarian by Athens) but at best they would form coalitions for a specific reason (say to fight Agamemnon's battles or to fight Darius and Xerxes. Ironically, Alexander the barbarian is the one who really dreamed of Hellenic civilization, a concept that transcended the city-states and was based mainly on the commonality of religion, language and culture, and not blood lines per se.
Regarding the Slavs in the Balkans, at the turn of the 20th Century, there many more Bulgarians in all parts of Macedonia than we see now. After each of the following conflicts, the Bulgarian population in Macedonia shrank: the unsuccessful Ilinden rebellion against the Turks (1903), the Balkan Wars (especially the Second), World War I, the Greek-Turkish War, and World War II. In the Aegean Macedonia (current Greek province of Macedonia), the ratio of Bulgarians to Greeks reversed dramatically after the great population exchanges in the early 1920s. This was helped along artificially by the declaration by the Greek authorities that no Bulgarians lived there but Greeks speaking a Slavic language, and discrimination and even persecution of Bulgarians who persisted in calling themselves that. It is clear though that, by far the greatest determinant of the current ethnic composition in the province of Macedonia was the influx of ethnic Greeks from Asia Minor in the 1920s.
The population of the Republic of Macedonia is currently little over 2 million souls (ranked 142nd in the world) in a 9,800 square mile area (148th in the world). Thus, it is smaller in size and population than the Greek province (about 500, 000 people less), but larger than about 50 other members of the United Nations. It has the same population as Slovenia and is larger than Estonia and Cyprus.
You are entirely correct that the Republic of Macedonia's culture, language, church and even history were helped along (and at times mandated) by the anti-Serbian ethnic groups in Yugoslavia--that is, through the Communist Party of the federation. Non-Serbians living in Macedonia, Croatia and Bosnia kind of ganged up on the Serbians, largely to free themselves from their influence and, at times, heavy handed rule.
As for the Macedonian Orthodox Church, in its form as the autonomous Archbishopric of Ohrid (1959-1967), it was recognised by the Serbian Church. The Wiki account is pretty accurate: "The Serbian Orthodox Church agreed with these decisions in the resolution AS. No 47/1959 and 6/1959, minutes 57 of June 17/4, 1959. That agreement was celebrated in a common liturgy by the Macedonian priests and the Serbian Patriarch German in 1959 in Skopje, as a sign that Serbian church recognises an autonomy of the Macedonian church. In 1962 Serbian Patriarch German and Russian Patriarch Alexis visited the Macedonian Orthodox Church. On the feast of Saints Methodius and Cyril in Ohrid two patriarchs and the Macedonian Metropolitan Dositej concelebrated Holy Liturgy as the first liturgy of the head of the Macedonian church with heads of other Orthodox churches."
Hope this adds to our understanding of the complex issues.