Conversation so far:
What was Dana's character like?
her Catholic faith served as a cornerstone.
What are some personality traits?
rigid skepticism.
Is there anything else interesting?
she was abducted by Duane Barry.
What happened after that abduction?
Mulder believes that Scully was taken aboard an alien spaceship and was subjected to tests.

Question: Why did she believe that?
Self-contained rewrite: Why did Mulder believe that Scully was taken aboard an alien spaceship and was subjected to tests?