How do Christians discern which parts of scripture are to be understood as myth (fictions with teaching/revelatory value) and which parts literally happened?
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None is myth. But I understand your question....
The best I can say is that the bible defines itself....
IF one permits it to do so. Almost ALL of the prophecies/depictions have dual definitions/fullfilments. In the case of prophecies; there is an ancient and a latter fulfillment. In the case of the depictions, such as David vs Goliath, there is both a worldly (material world meanings) as well as a Spiritual definition. Most understand the material meanings, but only certain understand the Spiritual definitions.
Also, the books of the Bible repeat, but in /from different view points. Like mathew, Mark, luke, and John. Each of the other books does the same; each reinforcing, adding, and building upon that which the previous book taught.
For instance... MOST think that only Adam and Eve had the choice of following God or following Satan....determining the course of history for future generations. Yet that is not what is taught...man has had repeated opportunities to set his world parallel to the way of God, he just never recognized them nor took them.
As for the symbolism,...again, that is defined through study/desire for truth....ALL the definitions are there...one simply has to seek them.
There are no myths in the Bible, except the ones man has put there. Get yourself a Concordence, an older KJV with center column or side margin references, and an Interlinear or Hewbrew, Greek, English Bible withe the words numbered and coresponding to Strong's. A good Bible eycyclopedia and a couple of Commentaries don't hurt either.
....oh, .... and don't forget to ask for understanding from God, ...without that and the proper attitude you'll just be chasing your tale or someone else's.