Here's what Barnes says (Barnes New Testament Notes):
Verse 18. Here is wisdom. That is, in what is stated respecting the name and the number of the name of the beast. The idea is, either that there would be need of peculiar sagacity in determining what the "number" of the "beast" or of his "name" was, or that peculiar "wisdom" was shown by the fact that the number could be thus expressed. The language used in the verse would lead the reader to suppose that the attempt to make out the "number" was not absolutely hopeless, but that the number was so far enigmatical as to require much skill in determining its meaning. It may also be implied that, for some reason, there was true "wisdom" in designating the name by this number, either because a more direct and explicit statement might expose him who made it to persecution, and it showed practical wisdom thus to guard against this danger; or because there was "wisdom" or skill shown in the fact that a number could be found which would thus correspond with the name. On either of these suppositions, peculiar wisdom would be required in deciphering its meaning.
Let him that hath understanding. Implying
(a) that it was practicable to "count the number of the name;" and
(b) that it would require uncommon skill to do it. It could not be successfully attempted by all; but still there were those who might do it. This is such language as would be used respecting some difficult matter, but where there was hope that, by diligent application of the mind, and by the exercise of a sound understanding, there would be a prospect of success.
Count the number of the beast. In #Re 13:16, it is "the number of his name." The word here rendered "count"? qhfisatw?means, properly, to count or reckon with pebbles, or counters; then to reckon, to estimate. The word here means compute; that is, ascertain the exact import of the number, so as to identify the beast. The "number" is that which is immediately specified, "six hundred threescore and six"?666. The phrase "the number of the beast" means, that somehow this number was so connected with the beast, or would so represent its name or character, that the "beast" would be identified by its proper application. The mention in #Re 13:17 of "the name of the beast," and "the number of his name," shows that this "number" was somehow connected with his proper designation, so that by this he would be identified. The plain meaning is, that the number 666 would be so connected with his name, or with that which would properly designate him, that it could be determined who was meant by finding that number in his name or in his proper designation. This is the exercise of the skill or wisdom to which the writer here refers: substantially that which is required in the solution of a riddle or a conundrum. If it should be said here that this is undignified and unworthy of an inspired book, it may be replied
(a) that there might be some important reason why the name or designation should not be more plainly made;
(b) that it was important, nevertheless, that it should be so made that it would be possible to ascertain who was referred to;
(c) that this should be done only in some way which would involve the principle of the enigma?"where a known thing was concealed under obscure language"?Webster?s Dic.;
(d) that the use of symbols, emblems, hieroglyphics, and riddles was common in the early periods of the world; and
(e) that it was no uncommon thing in ancient times, as it is in modern, to test the capacity and skill of men by their ability to unfold the meaning of proverbs, riddles, and dark sayings. Compare the riddle of Samson, #Jud 14:12, seq. See also #Ps 49:4 78:2 Eze 17:2-8 #Pr 1:2-6 Da 8:23 It would be a sufficient vindication of the method adopted here if it was certain or probable that a direct and explicit statement of what was meant would have been attended with immediate danger, and if the object could be secured by an enigmatical form.
For it is the number of a man. Various interpretations of this have been proposed. Clericus renders it, "The number is small, or not such as cannot be estimated by a man." Rosenmuller, "The number indicates a man, or a certain race of men." Prof. Stuart, "The number is to be computed more humano, not more angelico;"" it is a man?s number." De Wette, "It is such a number as is commonly reckoned or designated by men." Other interpretations may be seen in Poole?s Synopsis. That which is proposed by Rosenmuller, however, meets all the circumstances of the case. The idea is, evidently, that the number indicates or refers to a certain man, or order of men. It does not pertain to a brute, or to angelic beings. Thus it would be understood by one merely interpreting the language, and thus the connexion demands.
And his number is six hundred threescore and six. The number of his name, #Re 13:17. This cannot be supposed to mean that his name would be composed of six hundred and sixty-six letters; and it must, therefore, mean that somehow the number 666 would be expressed by his name in some well-understood method of computation. The number here?six hundred and sixty-six?is, in Walton?s Polyglott, written out in full: Exakosioi exakonta ex. In Wetstein, Griesbach, Hahn, Tittmann, and the common Greek text, it is expressed by the characters cxv= 666. There can be no doubt that this is the correct number, though, in the time of Ireneaus, there was in some copies another reading?civ= 616. This reading was adopted by the expositor Tychonius; but against this, Ireneaus inveighs.?Lib, v. c. 30. There can be no doubt that the number 666 is the correct reading, though it would seem that this was sometimes expressed in letters, and sometimes written in full. Wetstein supposes that both methods were used by John; that in the first copy of his book he used the letters, and in a subsequent copy wrote it in full. This inquiry is not of material consequence.
It need not be said that much has been written on this mysterious "number," and that very different theories have been adopted in regard to its application. For the views which have been entertained on the subject, the reader may consult, with advantage, the article in Calmet?s Dic., under the word Antichrist. It was natural for Calmet, being a Roman Catholic, to endeavour to show that the interpretations have been so various, that there could be no certainty in the application, and especially in the common application to the Papacy. In endeavouring to ascertain the meaning of the passage, the following general remarks may be made, as containing the result of the investigation thus far:
(a) There was some mystery in the matter?some designed concealment?some reason why a more explicit statement was not adopted. The reason of this is not stated; but it may not be improper to suppose that it arose from something in the circumstances of the writer, and that the adoption of this enigmatical expression was designed to avoid some peril to which he or others might be exposed if there were a more explicit statement.
(b) It is implied, nevertheless, that it could be understood; that is, that the meaning was not so obscure that, by proper study, the designed reference could not be ascertained without material danger of error.
(c) It required skill to do this; either natural sagacity, or particular skill in interpreting hieroglyphics and symbols, or uncommon spiritual discernment.
(d) Some man, or order of men, is referred to that could properly be designated in this manner.
(e) The method of designating persons obscurely by a reference to the numerical signification of the letters in their names was not very uncommon, and was one that was not unlikely, in the circumstances of the case, to have been resorted to by John. "Thus, among the Pagans, the Egyptian mystics spoke of Mercury, or Thouth, under the name 1218, because the Greek letters composing the word Thouth, when estimated by their numerical value, together made up that number. By others, Jupiter was invoked under the mystical number 717; because the letters of ?H APXH?Beginning, or First Origin, which was a characteristic of the supreme deity worshipped as Jupiter, made up that number. And Apollo under the number 608, as being that of huv or uhv, words expressing certain solar attributes. Again, the pseudo-Christian or semi-Pagan Gnostics, from St. John?s time and downwards, affixed to their gems and amulets, of which multitudes remain to the present day, the mystic word abrasax abrasax or abraxav abraxas under the idea of some magic virtue attaching to its number 365, as being that of the days of the annual solar circle," etc. See other instances referred to in Elliott, iii. 205. These facts show that John would not be unlikely to adopt some such method of expressing a sentiment which it was designed should be obscure in form, but possible to be understood. It should be added here, that this was more common among the Jews than among any other people.
(f) It seems clear that some Greek word is here referred to, and that the mystic number is to be found in some word of that language. The reasons for this opinion are these:
(1) John was writing in Greek, and it is most natural to suppose that this would be the reference;
(2) he expected that his book would be read by those who understood the Greek language, and it would have been unnatural to have increased the perplexity in understanding what he referred to by introducing a word of a foreign language;
(3) the first and last letters of the Greek alphabet, and not those of the Hebrew, are expressly selected by the Saviour, to denote his eternity?
"I am Alpha and Omega," #Re 1:8,11; and
(4) the numerals by which the enigma is expressed?cxv?are Greek. It has indeed been supposed by many that the solution is to be found in the Hebrew language, but these reasons seem to me to show conclusively that we are to look for the solution in some Greek word.
The question now is, whether there is any word which corresponds with these conditions, and which would naturally be referred to by John in this manner. The exposition thus far has led us to suppose that the Papacy in some form is referred to; and the inquiry now is, whether there is any word which is so certain and determinate as to make it probable that John meant to designate that. The word Lateinov?Lateinos, the Latin Man?actually has all the conditions supposed in the interpretation of this passage. From this word the number specified?666?is made out as follows:?
D A T E I N O S
30 1 300 5 10 50 70 200= 666.
In support of the opinion that this is the word intended to be referred to, the following suggestions may be made:
(a) It is a Greek word.
(b) It expresses the exact number, and corresponds in this respect with the language used by John.
(c) It was early suggested as the probable meaning, and by those who lived near the time of John; who were intimately acquainted with the Greek language; and who may be supposed to have been familiar with this mode of writing. Thus it was suggested by Irenaeus, who says, "It seems to me very probable; for this is a name of the last of Daniel?s four kingdoms; they being Latins that now reign." It is true that he also mentions two other words as those which may be meant?euanyav, a word which had been suggested by others, but concerning which he makes no remarks and which, of course, must have been destitute of any probability in his view; and teitan; which he thinks has the clearest claims for admission?though he speaks of the word Lateinos as having a claim of probability.
(d) This word would properly denote the Roman power, or the then Latin power, and would refer to that dominion as a Latin dominion?as it properly was; and if it be supposed that it was intended to refer to that, and, at the same time, that there should be some degree of obscurity about it, this would be more likely to be selected than the word Roman, which was better known; and
(e) there was a special propriety in this on the supposition that it was intended to refer to the Papal Latin power. The most appropriate appellation, if it was designed to refer to Rome as a civil power, would undoubtedly have been the word Roman; but if it was intended to refer to the ecclesiastical power, or to the Papacy, this is the very word to express the idea. In earlier times the more common appellation was Roman. This continued until the separation of the Eastern and Western empires, when the Eastern was called the Greek, and the Western the Latin; or when the Eastern empire assumed the name of Roman, and affixed to the Western kingdoms one and all that were connected with Rome the appellation of Latin. This appellation, originally applied to the language only, was adopted by the Western kingdoms, and came to be that by which they were best designated. It was the Latin world, the Latin kingdom, the Latin church, the Latin patriarch, the Latin clergy, the Latin councils. To use Dr. Mores words, "They Latinize everything: mass, prayers, hymns, litanies, canons, decretals, bulls, are conceived in Latin. The Papal councils speak in Latin, women themselves pray in Latin. The Scriptures are read in no other language under the Papacy than Latin. In short, all things are Latin." With what propriety, then, might John, under the influence of inspiration, speak, in this enigmatical manner, of the new power that was symbolized by the beast as Latin.
The only objection to this solution that has been suggested is that the orthography of the Greek word is latinov?Latinos?and not lateinov?Lateinos?giving the number 616, and not 666; and Bellarmine asserts that this is the uniform method of spelling in Greek authors. All that is necessary in reply to this, is to copy the following remark from Prof. Stuart, vol. it. p. 456: "As to the form of the Greek word lateinov Lateinos, viz., that ei** is employed for the Latin long i it is a sufficient vindication of it to cite sabeinov, fausteinov, pauleinov, lntwneinov, lteiliov, meteiliov, papeeriov, oueibiov, etc. Or we may refer to the custom of the more ancient Latin, as in Plautus, of writing i by ei; e.g., solitei, Diveis, captivei, preimus, Lateina, etc." See this point examined further, in Elliott, iii. 210-213.
As a matter of historical interest, it may be observed that the solution of the difficulty has been sought in numerous other words, and the friends of the Papacy, and the enemies of the Bible, have endeavoured to show that such terms are so numerous that there can be no certainty in the application. Thus Calmet, (Dic., art. Antichrist,) after enumerating many of these terms, says, "The number 666 is found in names the most sacred, the most opposite to Antichrist. The wisest and best way is to be silent."
We have seen that, besides the name Lateinos, two other words had been referred to in the time of Irenaeus. Some of the words in which the mysterious number has been since supposed to be found are the following:?
HEBREW Neron Caesar = 50 + 200 + 6 + 50, and 100 + 60 + 200 = 666
Diocles Augustus (Dioclesian) = ............................ DCLXVI.
C. F. Julianus Ceasar Atheus (the Apostate) = ................ DCLXVI.
Luther?Hebrew = 200 + 400 + 30 + 6 + 30 = ................ 666
Lampetis, lampetiv = 30 + 1 + 40 + 80 + 5 + 300 + 10 + 200 = 666
h latinh basileia = 8 + 30 + 1 + 300 + 10 + 50 + 8 + 2 + 1
+ 200 + 10 + 30 + 5 + 10 + 1 = ......... 666
italika ekklhsia = 10 + 300 + 1 + 30 + 10 + 20 + 1 + 5 + 20
+ 20 + 30 + 8 + 200 + 10 + 1 = ......... 666
lpostathv (the Apostate) = 1 + 80 + 70 + 6 + 1 + 300
+ 8 + 200 = ............................ 666
Hebrew (Roman, sc. Sedes) = 200 + 6 + 40 + 10 + 10 + 400 = 666
Hebrew (Romanus, sc. Man) = 200 + 40 + 70 + 50 + 6 + 300 = 666
It will be admitted that many of these, and others that might be named, are fanciful, and perhaps had their origin in a determination, on the one hand, to find Rome referred to somehow, or in a determination, on the other hand, equally strong, not to find this; but still it is remarkable how many of the most obvious solutions refer to Rome and the Papacy. But the mind need not be distracted, nor need doubt be thrown over the subject, by the number of the solutions proposed. They show the restless character of the human mind, and the ingenuity of men; but this should not be allowed to bring into doubt a solution that is simple and natural, and that meets all the circumstances of the case. Such a solution, I believe, is found in the word lateinov?Lateinos, as illustrated above; and as that, if correct, settles the case, it is unnecessary to pursue the matter further. Those who are disposed to do so, however, may find ample illustration in Calmer, Dict., Art. Antichrist; Elliott, Horoe Apoca. iii. 207-221; Prof. Stuart, Com. vol. ii., Excursus, iv.; Bibiotheca Sacra, i. 84-86; Robert Fleming on the Rise and Fall of the Papacy, 28, seq.; De Wette, Exegetisches Handbuch, 37. T., iii. 140-142; Vitringa, Com. 625-637, Excursus, iv.; Nov. Tes. Edi. Koppianoe, vol. x. b, pp. 235-265; and the Commentaries generally.