BtVS had more symbolism & intelligence in the 2 part series premiere than some shows have in their entire series run.
The herione/titular lead is from LA and it's important to the story because it's the City of Angels, it's "the town of our Lady the Queen of Angels." That sets the tone of who Buffy is more than just she's a big city girl moving to a one Starbucks town, though that is important too.
Angel gave the herione a cross necklace and said, "Don't turn your back on this." Later we find out he viewed her as his salvation and redemption before moving to her hometown to make a career out of saving souls and helping the hopeless. When he told her his name there was an angel statue right behind him because he saw himself as her guardian angel (watches over her, wants to protect her without interfering), a messenger (gives info on the paranormal and upcoming events) which is the etymology of angel, and metaphorically as a demon he is a fallen angel. Later she became a fallen angel when she was cast out of Heaven and back to the Hellmouth by Osiris (God of the dead, ruler of the Underworld).
The Master is an inversion of God. The name itself makes you think Lord. He has disciples, is given blood offerings/human sacrifices, is trapped in an underground church, and quotes from a book of prophecy as though it were scripture. He was referred to as The Sleeper and it connects him to Buffy since the first scene showing her she's asleep having a prophetic dream.
Buffy's first day in history class is about the black plague. Later Luke describes humanity as "a plague of boils" that covers the earth, wanting a return to the days of the Old Ones. Pureblood demons, like the fallen angels from religion, were cast out and sent to Hell dimensions.
The use of black & white in wardrobes was done on Supernatural for the angels and demons. Angel & Darla both wore black jackets over a white collared shirt. Buffy switched from white to black to white again. The Master wore all black.
Buffy entered the library looking for a history textbook and her new Watcher Giles over eagerly takes out the Vampyr book and she said it was not what she was looking for. It was reminiscent of Jehovah's Witnesses asking "Have you heard the good news?" And that night Buffy holds up an old fashioned dress and says, "Would you like a copy of The Watchtower?" Her mom asked if there would be boys there and Buffy said, "No mom, it's a nun's club." We find out later that Angelus had a thing for nuns and bad girls, basically the old Madonna whore complex. At this point Buffy is still a virgin but partially inspired by the promiscuous girls of horror movies who are punished for their sexuality by being killed early while the Final Girl is the one who didn't have sex.
When Buffy was deciding what to wear she was navigating societal expectations on female sexuality. She held up a short black dress, said "Hi, I'm an enormous slut" and then a very modest dress and compared it to being devoutly religious. In the end she went with pants.
It ties into actual ex prostitute, possible rape victim, and definite rape enabler (she used to bring Angelus girls) Darla (sired/damned by The Master) in a Catholic school girl uniform, using the image of girly innocence to lure boys (who want to exploit it for sexual gain) to their death & dammation.