In Revelation 13:18, six hundred and sixty-six is the gematrial number of the name of the individual man commonly called the Antichrist (the beast): "the number of his name. Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six" (Revelation 13:17-18); "the number of his name" (Revelation 15:2).
Gematria is an ancient system whereby the letters of an alphabet are assigned numerical values, as follows: the first nine letters of an alphabet are one through nine; the next nine letters are ten through ninety, counting by tens; and the rest of the letters in the alphabet are one hundred, two hundred, three hundred, etc., until the end of the alphabet.
So the number of a man's name is the sum of the gematrial numerical values of the letters in his name. For example, the number of the name "Fox" happens to be six hundred and sixty-six, because F=6, O=60, and X=600. This is not to say that the name of the Antichrist has to be "Fox", for many other names add up to 666 as well. For example, the name "Nabil Abdullah Falakal al-Hakim" adds up to 666. In the case of the Antichrist, it could be his full name, like this, that we will find adds up to 666, instead of just his first or last name, like "Fox".
The numerical values of all of the letters in the English alphabet are as follows:
A=1
B=2
C=3
D=4
E=5
F=6
G=7
H=8
I=9
J=10
K=20
L=30
M=40
N=50
O=60
P=70
Q=80
R=90
S=100
T=200
U=300
V=400
W=500
X=600
Y=700
Z=800
We may need to use the English alphabet when checking to see if some world leader's name adds up to 666. For the book of Revelation, like the rest of the New Testament, was originally written in ancient Greek only because that was the most common language of believers at the time that the New Testament was written. Similarly, the Old Testament was originally written in ancient Hebrew because that was the most common language of believers during the time that the Old Testament was written. Today, the most common language able to be understood by believers is English, which has become the lingua franca of the modern world. So it would make sense that the calculation that believers must perform, referred to in Revelation 13:18, would be performed using the English language. Also, transliterating modern names back into ancient Greek, or Hebrew, is a very inexact science, so we would never be certain that we had transliterated the modern name back into the ancient language correctly, that is, as God would. Also, we could have fiddled with the transliteration until it added up to 666. But if we stick with the modern name's standard English spelling, derived from standard written news sources in English like the New York Times and Time Magazine, we can go with their standard English spelling of the name of the world leader we are checking on to see if the number of his name adds up to 666, and we can avoid any weird, variant spelling of his name which some web site may have come up with by fiddling with his name until it added up to 666.
Of course, we should never discount anyone as not possibly being the Antichrist just because his name doesn't add up to 666 when the English alphabet is employed. For it could be the case that the Antichrist is an Arab, from Tyre, Lebanon (Ezekiel 28:2, cf. 2 Thessalonians 2:4), and his name may add up to 666 only when his native Arabic alphabet is employed. In this case, it will be up to Arabic-speaking Christians to alert their non-Arabic-speaking brothers and sisters in Christ that a certain Arab leader's name adds up to 666 in the Arabic alphabet.
Also, we should never claim that someone must be the Antichrist simply because the number of his name is 666, whether in the English alphabet, the Arabic, or any other. For the names of many different people in the world add up to 666, and they can't all be the Antichrist.
But if we do find that the number of a certain world leader's name adds up to 666, whether in the English alphabet or his own, native alphabet, we can then see if he passes other tests regarding his possibly being the Antichrist. Besides the number of his name adding up to 666, he must also come from a country the territory of which used to be part of one of the four Diadochian kingdoms which succeeded the Greek Empire of Alexander the Great (Daniel 8:8-9,21-25). So the Antichrist will probably come from the Middle East, and cannot come from the U.S., Western Europe, Russia, China, etc.
Also, while the Antichrist will probably come from the Middle East, the country he comes from must be "diverse" from the ten major endtime nations the territory of which used to be part of the Roman Empire (Daniel 7:24). The Roman Empire consisted not only of Western Europe, but also included the Middle East and North Africa. So the ten major endtime nations the territory of which used to be part of the Roman Empire could be the U.K., France, Germany, Spain, Italy, Turkey, Iraq, Syria, Egypt, and Algeria. The Antichrist has to come from a little country diverse from the ten major nations; he's even referred to as the "little horn" (Daniel 7:8, 8:9). In his rise to world power, he will first gain control of three of the ten major nations (Daniel 7:24).
The Antichrist could come out of the little country of Lebanon, from the modern city of Tyre (Ezekiel 28:2, cf. 2 Thessalonians 2:4), and first be given control of Syria, Iraq, and Egypt, which by that time could have been put together into a Baathist confederation. They could have been put together into a Baathist confederation by a predecessor of the Antichrist, an Iraqi Baathist General, after he defeated Israel and Egypt (Daniel 11:15-17; in verse 17, the original Hebrew word translated as "daughter" is "bath"). The Iraqi Baathist General could have defeated Israel and Egypt with a massive Iraqi Army built up by the U.S. to defeat Iran instead (with the idea of ending Iran's nuclear-weapons program and extremist regime). The Iraqi Baathist General could have invaded Israel in retaliation for the ultra-Orthodox Jews in Israel destroying the Dome of the Rock and the Al Aqsa Mosque on the Temple Mount in Jerusalem (to clear the Mount for the building of a third Jewish temple). After defeating Israel, the Iraqi Baathist General could have continued south and invaded Egypt because the Baathists hate the current Egyptian regime; they see it as the bought puppet of the U.S. The Baathists want to unite all Arab nations from Oman to Morocco into one massive United Arab States, or Arab Union, completely free from all Western hegemony.
Once the Antichrist gains control of a three-nation Baathist confederation (consisting of Iraq, Syria, and Egypt), he could gradually and peacefully extend the confederation (Daniel 11:24) until it includes all of the Arab nations. He could then convince an oil-thirsty European Union to let the oil-rich Arab Union join it, thereby forming a massive Mediterranean Union, which the Antichrist could then gain control of and use as his base of power to exert his hegemony over the entire world (Revelation 13:7).
The ultimate, and the unmistakable sign, that the Antichrist has arrived on the world scene, will be his sitting in a third Jewish temple and proclaiming himself God (2 Thessalonians 2:4, Daniel 11:36). He will do this after he has attacked the temple, stopped the daily Mosaic sacrifices offered in front of it, and placed the abomination of desolation (a standing image of himself) in the holy place of the temple (Daniel 11:31, Matthew 24:15).
