“What makes a religion a mythological one (Norse,Greek, etc.) or a bona fide one (Christianity, Islam)?”
Public opinion.
Norse and Greek mythology have very little basis in historical fact. As I’ve pointed out in previous points, there is some basis, but very little.
However, there are other dead religions that had historical basis. For example, Zoroastrianism. We know a great deal about him and his historical background. His theology was very mythical, however, and based on direct gnosis, not historical truth.
You can probably say that many early religions from paganism were based on tribal stories to explain nature and the structure of society. Everything in the ancient world was seen in a spiritual sense. For example, the Romans had gods for every consceivable action, and object you can imagine. There were gods for various social occasions. The emperor himself was seen as a god (or at least his spiritual side was seen as godlike, not his physical).
Judaism, on the other hand, tracks the historical progression of the people of God, who trace their ancestry back to Abraham. There are mythical elements in parts of the narrative, but most of what is written can be traced back to real historical sources using archaeology and corroborating historical records from other cultures and religious groups of the day.
There is still much debate on whether Abraham lived, but most scholars believe that his story has a basis in historical fact. The same goes for Moses. Kind David and most of the kings of Judah and Israel are amply attested to in other writings. Herodotus provides us with a Greek perspective on many of the events listed in the book of Kings and Chronicles, including several battles, and the rise and fall of the Babylonians, Cyrus, etc…
This is defnitely not myth. Even secular historians would never call the history of Israel myth. You can dispute the religious experiences described in the Bible, but you can’t call these experiences mythology.
The same thing goes for Christianity. The earliest writings we have are from Paul from around 49 AD. There are a plethora of other early Christian sources in the Bible and outside of the Bible. You also have mention of Christianity in Pliny, Josephus, and Tacitus, to name a few. All of these attest to the historicity of Christ and his followers.
Even if Christianity were to dissappear tomorrow, no one would call Christianity a myth.
God bless,
Ut