Complex sentence: Applewhite believed in the ancient astronaut theory, that extraterrestrials had visited humanity in the past and placed humans on earth, and that they would return to collect a select few.
Split into simpler sentences: Applewhite believed in the ancient astronaut theory, that extraterrestrials had visited humanity in the past. He taught that extraterrestrials had placed humans on earth, and that they would return to collect a select few.