The lore of BB isn't that hard, i mean the game doesn't literally give you a guy that explains everything like other RPGs, but it's not as hidden as souls games.
We can easily lay down the events one after the other:
1-In ancient times, a race known as the Pthumerians was tasked by the Great Ones, lovecraftian otherworldy creatures, to guard the huge labiryinths (the Chalice Dungeons) serving as their homes/tombs/wahtever.
2-They started out as just guardians but became corrupt and evil, started using for power and using the old blood of the Great Ones for their own gain, Queen Yharnam was apparently especially cruel during her reign.
3-Master Willem and Laurence find the Chalice Dungeons under Byrgenwerth, and they start sending in people to explore it, they are scholars of the Institute, a group dedicated to learning and research.
4-They find the Great Ones down there, and their blood, the blood of the Great Ones has a number of properties, it can heal wounds and cure diseases and make you stronger and so on.
5-Willem and Laurence both think they're on the verge of unlocking the hidden potential within humanity and evolve to the next stage of human evolution into something greater, but they disagree on what path to take, Laurence wants to use the blood of the Old Ones, while Willem wants to line mankind brain with eyes and explore insight.
6-The two part ways, Willem remains with the Institute, while Laurence starts the Church, a group dedicated to study and use the blood.
6-Willem focuses his research on lining the brain with eyes and insight, on his own, surrounded by loyal students.
7-Laurence focuses his research on using the blood, and the church slowly starts becoming huge because they're an organization that now posseses an all-healing substance that can cure most illnesses.
8-Yharnam, the city, becomes rich and famous because the Church is there, the Church now controls pratically everything and it's huge.
9-Using the blood starts having side effects, people start to become sick after abusing it.
10-People that got sick starts turning into fucking beasts, the Church starts recruiting people to hunt down the beasts, these are called Hunters, one of the first hunters is Gehrman, and his pupil, Maria.
11-Old Yharnam is burnt down to stop the spreading of the sickness.
12-At some point, i don't know when exactly so i'm gonna just put it here in the timeline, the Church finds a fishing village, and near that village they find a Great One, they do SOMETHING to that Great One, it's not clear what, something concerning her child, the Church used the earliest hunters to do this, including Maria, and the entire village becomes cursed, so are all the hunters that die after becoming frenzied abusing the blood during their hunts, Maria is disgusted by whatever they had to do back there, and kills herself.