Eusebius of Caesarea

Without Eusebius we wouldn't know early Christianity. His Ecclesiastical History is the source for almost every story about the first three centuries — apostolic succession lists, persecutions, heretics, Christian intellectual life — and he had access to the libraries of Caesarea and Jerusalem that we lost. Modern historians constantly second-guess him (he had agendas) but he's the only door we have. He also baptised the dying Constantine. Christianity's transition from persecuted sect to imperial religion runs through one man's library.