It is a Latin phrase that is attributed to Constantine the Great, the first Christian Emperor of Rome. According to legend, Constantine saw a vision of a flaming cross in the sky before the Battle of the Milvian Bridge in 312 AD. The cross bore the words "In hoc signo vinces," which Constantine took as a sign that he would be victorious in the battle. He went on to defeat his rival, Maxentius, and became the sole emperor of the Roman Empire.
The phrase "In hoc signo vinces" has been used as a motto by many different people and organizations throughout history, including the Knights Templar, the Spanish Inquisition, and the Ku Klux Klan. It is also the state motto of Georgia and the name of a football club in Italy.