I think the argument about the origin of the number 666 being DCLXVI in Roman numerals would make a certain amount of sense if it weren't for one thing: the book of Revelation was written in Greek, not Latin.
The oldest decryption of this verse takes a similar approach, though. It dates back to the second century, when Irenaeus, a disciple of Polycarp who was in turn a disciple of John the Apostle (the author of Revelation), used the Greek system of using letters to stand for numbers to decode 666 as Lateinos:
L = 30
A = 1
T = 300
E = 5
I = 10
N = 50
O = 70
S = 200
-------
666
...
Lateinos being the Greek spelling of Latinus, the reputed founder of the Latin race. Since
Latin was, of course, the official language of the Roman Church, Irenaeus took this to mean that the
Pope of the
Roman Catholic Church was to be the mysterious beast.