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In this talk Armand Newrick starts in Genesis and looks at how the concept of Gehenna develops through the Bible. There's 26 minutes of teaching followed by 21 minutes of discussion.
You can see a transcript with comments and discussion at afterlife.co.nz/2012/theology/tracing-the-road-to-gehenna/
What is conditional immortality? The answer is at afterlife.co.nz/free-resources/what-is-conditional-immortality/