Diatribe: an angry and usually long speech or piece of writing that strongly criticizes someone or something

Source: http://www.merriam-webster.com/dictionary/diatribe?show=0&t=1386997887

“Will Self’s new novel, “The Book of Dave” is about a London cabdriver who inadvertently founds a religion when ranting diatribe he buries in the garden of his ex-wife.” from A Literary Visitor Strolls in From the Airport by Charles Mcgrath

In Will Self’s novel a piece of writing of a creator of a self created religion was dug up five centuries after the creator and the diatribe explains the components of the religion.