John II
Mercurius · 533 – 535
The first pope to change his name upon election, setting a precedent that continues today.
Mercurius was a Roman priest who, upon election in 533, took the name John II because his birth name was that of a pagan god. He was the first pope in history to adopt a new name, starting a custom that would become standard centuries later. He worked closely with Emperor Justinian on matters of doctrine and confirmed a theological formula about Christ's nature.
Why He Matters
Every time a new pope chooses a name — Francis, Benedict, John Paul — that tradition traces back to Mercurius deciding in 533 that a pope should not carry the name of a pagan god. A small practical decision became a lasting custom.