I thought I’d post this as a handy reference for “essential” reading to know the bases of the predominant religious philosophies in the world: The Platonic-Aristotelian traditions of the West, the Hindu traditions, and the Theravada/Tibetan Mahayan/East Asian Mahayana traditions.

It can be hard to wade through “everything”, but these are in my estimate, the most crucial elements.

This is not exhaustive, but it is comprehensive.

Platonic-Aristotelian:
The Presocratics
The Other Plato by Dmitri Nkulin (if you want to know what he meant underneath it all)
People of Plato by Debra Nails (essential to understand the dialogues)
Complete works of Plato
Basic Works of Aristotle
Plotinus (Gerson translation)
Greg Shaw on Iamblichus’ Theurgy
Porphyry’s “Letter to Anebo” and Iamblichus’ “De Mysteriis”
Proclus’ Commentaries on Alcibiades, Timaeus, Parmenides, and the Theology of Plato and Tria Opuscula

After all that, it’s basically just Judaeo-Christo-Islamic reflections on that fundamental architecture.
Augustine’s Confessions, City of God, On the Trinity, On Christian Doctrine, On Free Will and Grace, A Literal Reading of Genesis
Complete works of Dionysius the Areopagite
Classical Arabic Philosophy: An Anthology
Ibn Tufayl’s Hayy ibn Yaqzan
Maimonides’s Guide (Hackett)
Aquinas’ Summa Theologica
Dante’s Divine Comedy

The Bible and the Koran are best read in light of all of that. You can check out Luther’s 95 Theses and Calvin’s Institutes of the Christian Religion if you swing Protestant though, but to make sense of these works it’s still best to have read all of that first. If interested in the meditative traditions of Christianity as represented in the Eastern Orthodox traditions, the Philokalia is recommended (it is available in five volumes).

Hinduism:
The Rig Veda, the Upanisads, The Mahabharata, The Ramayana, The Maha Puranas
Mimamsa Sutra, Samkhya Sutra, Yoga Sutra, Vaisheshika Sutra, Nyaya Sutra, 3 vol. Vedanta Sutras

Jainism: Umasvati

Theravada: the Pali suttas, Bhikkhu Bodhi’s Comprehensive Manual on Abhidhamma, the Visuddhimagga

Indo-Tibetan Mahayana:

Garfield’s translation of Mulamadhyamakakarika, Asanga’s Mahayanasamgraha, Seven Works of Vasubandhu by Anacker, the two translated works by Candrakirti, Santideva’s Way of the Bodhisattva, Santaraksita’s Ornament of the Middle Way, Longchenpa on Dzogchen, the 9th Karmapa’s commentary on Moonbeams of Mahamudra, Dolpopa: The Buddha from Dolpo, Tsongkhapa’s Ocean of Reasoning, Gorampa on the Two Truths, Taranatha’s The Essence of Other-Emptiness

East Asian:

The Hackett translations of Confucius and Mencius
Brook Zipporyn’s Tao Te Ching and Zhuangzi (recommended with Zipporyn’s Zhuangzi: do both his translation of the inner chapters with commentary and the full work without the commentary), and AC Graham’s Liezi
Zipporyn’s book on Tiantai, and Cleary’s book on Huayan
Zen:
Red Pine’s Bodhidharma
Zen Dawn by Cleary
The Ceasing of Notions
Red Pine’s the Lankavatara Sutra and Diamond Sutra
The Platform Sutra (Cleary or Red Pine)
Guifeng Zongmi
Five Houses of Zen (Cleary!
The Record of Linji by Ruth Fuller
Yongming Yanshou
Hongzhi’s Silent Illumination
Swamp Letters by Ta Hui
Chinul Tracing Back the Radiance
Dogen’s Shobogenzo
Hakuin’s Poison Blossoms