•Why it earns a slot
The Age of Reason is the work that forced organized Christianity to defend its scriptures on evidential grounds rather than authority alone, sparked thirty years of prosecutions for blasphous libel in Britain, and pioneered the textual and historical methods of biblical criticism that mainstream scholarship would not openly adopt for another century.
Written under threat of the guillotine in revolutionary Paris, The Age of Reason is Paine's systematic case against revealed religion and in favor of Deism. Part One argues that the only true word of God is the creation itself, that all national churches are human inventions built on hearsay and fraud, and that Jesus was a virtuous moral teacher whose supernatural biography was borrowed from pagan mythology. Part Two, written while Paine was a prisoner in the Luxembourg, subjects the Old and New Testaments to detailed textual and historical scrutiny, concluding book by book that the named authors could not have written the works attributed to them, that the narratives are riddled with contradictions and fabrications, and that the only rational religion is the pure Deism grounded in reason and the observable universe.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.