Peter Brown, that great historian of early Christianity, has given the most cogent explanation for the arising of the cult of the saints in the late Roman world. He explains that the emphasis of early Christian preaching on judgment, on the human need for redemption from sin, brought to the minds of common people — among whom Christianity was early successful — their social and political condition. Having strictly limited powers to remedy any injustice they might suffer, or to clear themselves of any charges of wrongdoing, they turned, when they could, to their social betters in hope of aid. If a local patrician could befriend them — could be, at least for a time, their patron — then they had a chance, at least, of receiving justice or at least escaping punishment. “It is this hope of amnesty,” Brown writes, “that pushed the saint to the foreground as patronus. For patronage and friendship derived their appeal from a proven ability to render malleable seemingly inexorable processes, and to bridge with the warm breath of personal acquaintance the great distances of the late-Roman social world. In a world so sternly organized around sin and justice, patrocimium [patronage] and amicitia [friendship] provided a much-needed language of amnesty.”
As this cult became more and more deeply entrenched in the Christian life, it made sense for there to be, not just feast days for individual saints, but a day on which everyone’s indebtedness to the whole company of saints — gathered around the throne of God, pleading on our behalf — could be properly acknowledged. After all, we do not know who all the saints are: no doubt men and women of great holiness escaped the notice of their peers, but are known to God. They deserve our thanks, even if we cannot thank them by name. So the logic went: and a general celebration of the saints seems to have begun as early as the fourth century, though it would only be four hundred years later that Pope Gregory III would designate the first day of November as the Feast of All Saints.
— from my book Original Sin: A Cultural History