Religion refers to a series of different belief systems, in regards to god(s) or the afterlife. Elements from these religions, whether from the mythology or in the form of religious persons, are featured on ABC's Once Upon a Time and Once Upon a Time in Wonderland.
- Christianity is present in Camelot, shown when Sir Lancelot says "God in His mercy, lend her grace". Lancelot is also nicknamed "the Leviathan", which is a biblical creature.
- Christianity is also in the Land Without Color, as the Frankenstein family are seen celebrating Christmas.
- David was a shepherd-turned-prince who has a mother named Ruth and faces a giant.
- Catholicism is in Victorian England, as Edwin is shown to be a religious man who prays before meals.
- The presence of deities from Greek mythology are present, such as: Poseidon, Hades and Zeus.
- The presence of these deities seem to cement some sort of accuracy in regards to ancient Greek religion.
- Mount Olympus and the Underworld are also shown, but seem to allude to Heaven and Purgatory from the Bible, whilst also displaying a "Worse Place" that seems to allude to Hell.
- Isaac Heller is a Jew, having mentioned having a bar-mitzvah.
- In a part of the Enchanted Forest, a maritime kingdom worships a sea goddess named Ursula. The same goddess is worshipped by mermaids. Ursula is shown to be quite real, when she confronts Regina for impersonating her.
- The Holy Grail is considered by Adda and Merlin to a be "gift from the gods". It is, as of yet, uncertain which gods he was referring to.