One concept which is really an addition to the bible is the concept of hell. This first appears in the new testament which was compiled in the 4th century, the mid 300s.
Depending on which modified or edited version of the christian bible you use hell in various forms is mentioned around 20 different times.
https://carm.org/what-are-the-verses-that-mention-hell
Now "hell" isn't actually in the bible at all unless you get a hold of a bible version that has been specifically and purposely mistranslated to include the word "hell".
The versions of 'hell" are varied. "Sheol" paints hell as a grave or a place that the dead remain. It is a loose jewish translation of the Greek word Hades. But Hades is actually a Greek god that tends the dead as well as a place where the dead go.
The concept of hell actually originates from what westerners call the Egyptian Book of the Dead. The actual title is "The Book of Coming Forth by Day or Spells for Going Forth by Day". It was a manual for knowing how to navigate the netherworld. A person would buy such a manual from an Egyptian priest and the priest would write a new manual each time, no two being alike.
In the 4th century when the new testament was actually compiled edited and written, hell became more significant. After all the Emporer Constantine needed the threat to hold over people's heads to force them to obey his adopted religion.
So in REAL christianity stemming from the first century by word of mouth and random letters, there is no real hell concept, just life and death and eternal life and eternal death. The "fire and brimstone" was added much later to give the Emporer leverage on his people.
This is just one example of the biblical afterthought. There are many others like using "resurrection" a concept stolen from other religions to bolster the mystique of jesus the character In 323 ADE these "afterthoughts" additions were debated to create what we now call the New Testament. Which further points out the political nature and the lack of credibility of the bible.