You may be surprised at the answer.
The phrase is deeply engrained into Western culture. Whenever someone sneezes the immediate response from a nearby person is “God bless you,” or “Bless you!” Most of us never even think about it, and the phrase continues to be passed down to each generation.
Why do we say it? Where did it come from?
The phrase itself comes directly from the Old Testament and the practice of the early Christians. In the book of Numbers we see it rendered, “The Lord bless you and keep you!” (Numbers 6:24). Other instances are found in the liturgy, such as the phrase, “The Lord be with you.” It was a common phrase for Christians and used in many different circumstances.
The phrase wasn’t attached to sneezing until the 7th century. According to History.com, the phrase “derives from a papal decree supposedly issued during the reign of Pope Gregory I. Also known as Gregory the Great, he assumed the papacy in 590, at a time when the bubonic plague was raging through Europe. An early sign that the virus had entered a victim was a sneeze, so on or around 6 February 600 AD, the Pope is attributed with suggesting that God’s blessing be offered to anyone who sneezes in order to protect against falling ill.”
There was another tradition that stated a person is vulnerable to the devil when they sneeze, so saying “God bless you,” prevents any malevolent activity from occurring.
Regardless of the exact origins, the phrase is a biblical prayer that asks for God’s blessing upon someone who is suffering from sickness. It is a kind gesture, one that, if prayed with faith, calls down God’s grace upon someone in need.