The earliest extant record of a Protestant writer on this subject is that of Professor Andreas Helwig in 1612. In his work Antichristus Romanus he took fifteen titles in Hebrew, Greek, and Latin and computed their numerical equivalents in those languages,

The earliest extant record of a Protestant writer on this subject is that of Professor Andreas Helwig in 1612. In his work Antichristus Romanus he took fifteen titles in Hebrew, Greek, and Latin and computed their numerical equivalents in those languages, arriving at the number 666 mentioned in the Book of Revelation. Out of all these titles, he preferred to single out Vicarius Filii Dei, for the reason that it met "all the conditions which [Cardinal] Bellarmine had thus far demanded." Besides being in Latin, the title was "not offensive or vile," but rather was "honorable to this very one."

The papal tiara given to Pope Pius IX by Queen Isabella II of Spain in the 1850s.
The tiara contains no writing.
Helwig suggested that the supposed title was an expansion of the historical title Vicarius Christi, rather than an official title used by the Popes themselves. Additionally, he said nothing about the title appearing on tiaras or mitres. Helwig's interpretation did not become a common one until about the time of the French Revolution.[9] Some later Protestant figures directly claimed that Vicarius Filii Dei was an official title of the Roman Catholic Pope, some claimed that this title appeared on the papal tiara and/or a mitre.

Some Protestants view the Pope as the Antichrist. This view was common at the time of Helwig and is still part of the confession of faith of some Protestant churches, such as those within Confessional Lutheranism.[10]

Roman numerals
Some Protestants identify the Roman Papacy with the Number of the Beast (666) from the book of Revelation, and believe that the phrase Vicarius Filii Dei, reduced to its Roman numerals, sums up to 666, where "U" is taken as "V" (two forms of "V" developed in Latin, which were both used for its ancestor "U" and modern "V"). To produce 666, the sum works as follows: VICARIVS FILII DEI = 5+1+100+1+5+1+50+1+1+500+1 = 666.[citation needed]