At some point when watching X Files, either as a new person watching it fresh, or as an old fan of the series, you may wish to watch Quartermass and the Pit. It's a British science fiction film which might have influenced Chris Carter, perhaps unconsciously, for many of the themes of the X Files appear in that film.
It was serialized in the late fifties, and made into a film in the late sixties.
I can't paste the youtube link for the fifties version as it might break copyright. The copyright might have expired though.