Vicarius Filii Dei

Vicarius Filii Dei (Latin: Vicar or Representative of the Son of God) is a phrase used in the forged medieval Donation of Constantine to refer to Saint Peter. It also features in the argument put forth by some Protestant groups who identify the phrase with the "number of the beast" (666) from the book of Revelation and subsequently the Pope with Antichrist, based on the counting method (gematria) of Roman numerals.

There is a papal title in existence that means the same: "Vicarius Iesu Christi" or Vicar of Christ. This title as it is used does not fit the system that calculates 666 out of the Latin words.

Source: wikipedia.org