I'll start with the OT and the 'Jewish Bible' ( which Christians adopted ) ;
It is a collection of myth fables and some oral history and folk stories with some 'other histories' and their famous people 'adopted' and given other names ( like Solomon being based on Egyptian, Assyrian or Phoenician Kings ) collated contemporary with Josiah ( who made the Temple 'Jewish ' and Monotheist (it was not previously ) in an attempt (successful until recently* ) to create an idea of a previous unified kingdom between Israel and Judah , so Josiah could move north and take Israel due to the post Neo-Assyrian 'withdrawal / power vacuum in that area .
* due to modern research about the adoption , development and evolution of Jahweh into Judah and the work of modern archaeologists like the prominent Israeli Dr Israel Finklestein ( to name one ) .
In that regard , to me, I have to consider it one of the most successful con jobs in history ! ( and the modern world still has ideas about God from this source ! )
The NT is a collection of ancient, 1st-century Greco-Roman literature authored by humans , influenced by both Second Temple Judaism ( see above ) and Greco-Roman literature, with anonymous authors (not the apostles ), creating Jesus as the main character to hold the story plot together and to present the authors ethical and philosophical arguments about justice, mercy and subversion ( due to Roman rule ).
Generally ( NT + OT ) Its a great story and probably became so popular due to it being 'on the story line ' that is most popular ( Cinderaella has the same story line structure )