Irenaeus of Lyons, Against Heresies 5.30

All these facts are confirmed by every ancient, approved, and authoritative Scripture, and by the testimony of those who saw John face to face; reason itself also teaches that the number of the beast's name, calculated according to the Greek method through the letters that compose it, comes to six hundred sixty-six—that is, equal tens of hundreds and equal hundreds of units. The digit six, similarly preserved, displays the recapitulations of his apostasy: the one that began at the start, the one in the middle times, and the one that will occur at the end. I cannot understand how certain persons went astray by following a foolish error, mutilating the middle of the number of the name by subtracting fifty so that they could make a single decade stand for six decades. I judge this to have been a scribal sin, as often happens, because numbers are also expressed by letters, and the Greek letter that signifies sixty was easily expanded into the Greek iota. Afterward some accepted this without investigation; others simply and foolishly used the number ten; still others dared to search out names that contained the false number of error. To those who acted merely out of simplicity and without malice we believe God will grant pardon. But as for those who, for the sake of empty glory, establish names containing the false number and declare their own invented name to be that of the coming one—they will not escape loss, because they have deceived both themselves and those who believe them. The loss itself is to fall away from the truth and to treat what does not exist as real. Moreover, adding to or taking away from Scripture brings no small penalty, which such a person must inevitably incur. Another danger, by no means slight, will also follow those who falsely presume to know his name: if they think one thing while he arrives with another, they will easily be seduced by him, as though he were not yet present—the very one we ought to beware of.

Therefore such people must learn and return to the true number of the name, lest they be classed as false prophets. Knowing the firm number announced by Scripture—six hundred sixty-six—let them first endure the division of the kingdom into ten. Then, while those ten reign and begin to set their affairs in order and expand their dominion, the one who comes unexpectedly and claims the kingdom for himself will terrify those former rulers; having a name that contains the aforesaid number, he must be recognized as the abomination of desolation. The Apostle also says: when they say, "Peace and security," then sudden destruction will come upon them. Not only his sudden coming, but also the tribe of six by which he will come, is revealed: "The voice of the neighing of his horses will shake the whole earth; he will come and devour the earth and its fullness, the city and those who dwell in it." Because of this, that tribe is not counted among the saved in the Apocalypse. It is therefore more certain and without danger to await the fulfillment of prophecy than to speculate and guess at any names; for many names can be found that possess the predicted number, and yet the same question would remain. If many names are discovered bearing this number—the one the coming one will bear—it will still be asked which one is his.

We say these things not because of a shortage of names that have the number of his name, but out of reverence and zeal for the truth. The name Euanthas possesses the number in question, yet we affirm nothing about it. The name itself totals six hundred sixty-six, and it is very plausible, since this final kingdom bears that name; the Latins are the ones who now reign, but we do not boast in this. Also the name Titan, written with two Greek vowels e and i, contains the predicted number in itself and is worthy of confidence. It consists of six letters, with single syllables made of three letters each, and it is ancient and set apart; for none of the kings who belong to us has been called Titan, nor do the idols publicly worshiped among Greeks and barbarians bear that name. Many regard the name as divine, so that even the sun is called Titan by those who now hold power. The name also carries a certain display of vengeance and retribution, since he pretends to avenge those who have been mistreated. It is likewise ancient, trustworthy, and royal—yet more tyrannical. Because the name Titan carries such weight of persuasion and verisimilitude, we gather from many indications that perhaps the coming one will be called Titan; nevertheless we will not risk it, nor will we assertively pronounce that he will bear this name. We know that if it had been necessary to proclaim his name openly at the present time, it would have been published by the one who also saw the Apocalypse; for the vision was not seen long ago, but almost in our own century, at the end of Domitian's reign.

Now the number of the name is shown so that we may beware of the one who is coming, knowing who he is; but the name itself is kept silent, because it is not worthy to be proclaimed by the Holy Spirit. If it had been proclaimed by Him, perhaps it would have endured for a long time. But now, since the beast is not, yet will ascend from the abyss and go to perdition as though he did not exist, neither is his name proclaimed; for the name of that which is not is not proclaimed. When the Antichrist has established all things in this world, he will reign for three years and six months and will sit in the temple at Jerusalem. Then the Lord will come from heaven in clouds, in the glory of the Father, sending him and those who obey him into the lake of fire, but bringing the times of the kingdom to the just—that is, rest, sanctifying the seventh day—and restoring the promise of inheritance to Abraham. In that kingdom the Lord says many will come from east and west and will recline with Abraham, Isaac, and Jacob.