In an earlier time, the world's cultures rarely interacted.
Each developed its own religion, and there were many types of religion, from naturalistic ones to pantheons full of specialized deities. Most were isolated within a particular culture and followed only by people from that culture. Pretty much every culture on the planet developed a religious system of one sort or another.
As commerce and technology led to travel, exploration and trade, people got exposed to the religious beliefs of other cultures. In the Bronze/Iron Age, the Middle East was a hotbed of trade and cultural exchange. More and more people were exposed to different religions created by all of those cultures.
One culture's religion was another culture's primitive mythology. The Middle East, though, had been trading internally for quite some time before the Europeans got there. Monotheism, in various forms, was a common, if not exclusive, thread there. It had simplicity going for it, for one thing. Judaism got a strong foothold, followed by its offshoot, Christianity.
As the Romans and Greeks traded and invaded, that monotheistic trend may have seemed a lot simpler to manage than the oddball pantheon of deities, each with its own focus and places of worship. The time was right for simplification and trimming down the number of deities that needed to be worshiped and propitiated. There was business to do, and more. Who had time for all those strange dieties with such human characteristics, after all?
In just a few centuries, what had been the believed-in polytheism of Greece and Rome became mythology, with Christianity becoming the "true" religion. It was an amazing phenomenon. Over the next centuries, Christianity got carried here and there and around the world, bringing its simplicity and one-step salvation to compete with more complex religious beliefs.
Christianity conquered, followed by a close contender in Islam, which is really a splinter offshoot of the Judeo/Christian belief system. Asia remained unconverted for a very long time, and still retains its own cultural religions, but even those are under pressure.
One culture's religion is another culture's mythology, and vice versa. The religion of the dominant culture economically and militarily, though, tends to overwhelm that of less capable cultures. But still, one culture's mythology is another's religion. All, however, remain human-created belief systems. All are, in my view, constructs of human culture, rather than revealed truth.
And so it goes...
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Feet of clay and all that. 
