Two stories fused into one reflex. The older one: a sneeze could expel the soul or open a door to spirits, so bystanders spoke a blessing to seal the body shut, a belief found far beyond Europe. The famous one: Pope Gregory I, facing the plague of 590, ordered blessings after sneezes because sneezing was the first symptom. Neither story can be proven. Both are immortal.
Assessment: The soul-door belief is genuinely ancient but never written doctrine. Gregory did stage plague prayer processions, but the sneeze link is unproven legend. The custom is real; its origin story is a double myth.
Interesting claims
A micro-exorcism
Esoteric readings frame it as a vestigial exorcism: the sneeze momentarily breaks the boundary between body and spirit, and the spoken "bless" reseals it.
Assessment: A symbolic interpretation, not a historical claim.
Documented
Older than the plague
Romans responded to sneezes with a wish for Jupiter's protection, and the custom appears in records across two millennia. Old English bletsian meant to consecrate with blood; the blessing formula attached to sneezing long before any pope endorsed it.
Assessment: Documented as a custom of genuine antiquity, though no single origin moment survives. Every culture seems to have invented it independently, which is its own data point.
Sources and further reading
Pliny the Elder, Natural History 28.5, on sneezing customs
Jacobus de Voragine, Golden Legend 45, Gregory I's litanies in the Roman plague
Worth asking
Why does nearly every culture on earth feel a sneeze demands a spoken response?