On the Nature of the Gods (Latin: De Natura Deorum) is a dialogue by Cicero which examines theology from the point of view of various philosophical schools. In book 1, Gaius Velleius gives the Epicurean argument for the existence of God, while Quintus Lucilius puts the Stoic case in book 2. Gaius Cotta criticises both viewpoints in book 3 from the viewpoint of Cicero's own academic skepticism.
Free online texts
Gutenberg: Cicero's Tusculan Disputations also, Treatises On The Nature Of The Gods, And On The Commonwealth, translated by C.D. Yonge. Multiple formats.
Internet Archive: The treatises of M.T. Cicero: On the nature of the gods; On divination; On fate; On the republic; On the laws; and On standing for the consulship. Literally translated chiefly by the editor, C.D. Yonge (1878). Multiple formats.
Internet Archive: De natura deorum, De divinatione, De fato, edited by Reinholdus Klotz (1879). Latin text, multiple formats.
Latin Library: De Natura Deorum. Latin text, HTML format.
Online Library of Liberty: De Natura Deorum (On the Nature of the Gods), trans. Francis Brooks (London: Methuen, 1896). Multiple formats.
Perseus: De Natura Deorum. Latin text, HTML and XML format.
Wikisource: De Natura Deorum. Latin text, multiple formats.
Academia.edu: Cicero Handout - Arguments For And Against God's Existence in On the Nature of the Gods, by Gregory Sadler.
History of Philosophy without any gaps: Rhetorical Questions: Cicero - podcast by philosopher Peter Adamson.
History of Philosophy without any gaps: Raphael Woolf on Cicero - podcast by philosopher Peter Adamson.
Internet Encyclopedia of Philosophy: Cicero.
Librivox: On the Nature of the Gods - public domain audiobook.
The Great Conversation: Further reading at Tom's Learning Notes
Plato: Euthyphro - a classic examination of divine command theory.
Cicero: Tusculan Disputations - defends Stoic views on happiness.
Cicero: On the Ends of Good and Evil - discusses the ethical teachings of the major philosophical schools.
Cicero: Academica - dialogues on the theory of knowledge.
Cicero: On Divination.
Cicero: On Fate.
Latin resources: Learn to read Latin texts in the original.
Bloom's Western Canon: On the Nature of the Gods is listed.