Reason, the Only Oracle of Man; Or, A Compendius System of Natural Religion examines religion by submitting doctrine, scripture, miracles, and ecclesiastical authority to the court of rational inquiry. Allen argues that nature, not revelation, discloses God's existence and moral order. Its style is blunt, polemical, and forensic: less elegant than Hume or Voltaire, but vigorous in its American directness. Published in 1784, it stands among the earliest sustained expressions of American deism and reflects the Enlightenment's challenge to inherited orthodoxy. Ethan Allen, best known as the Revolutionary leader of the Green Mountain Boys and hero of Fort Ticonderoga, was also a freethinking political and religious radical. His frontier experience, resistance to imposed authority, and commitment to republican liberty shaped his suspicion of priestcraft and dogma. The book's intellectual independence mirrors the political independence for which he fought. This work is recommended for readers interested in early American thought, deism, religious controversy, and the intellectual aftermath of the Revolution. It rewards those willing to encounter a combative but historically important argument for reason as humanity's highest guide.