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Revelation 13:17 And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name. 18 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.
Six hundred threescore and six is written in Greek as ‘chi’ ‘xi’ ‘stigma’ which are the 22nd, 14th and an obsolete letter of the Greek alphabet (intermediate between the 5th and 6th), used as numbers; the meaning of which is the basis of much vain speculation.
Revelation is a book of prophecy given in symbols covering the period from John’s day through to the end of the millennial reign of Christ. The primer for understanding the symbol of the beast is in Daniel.
Daniel 2 in the dream of Nebuchadnezzar’s image gives an outline of human history through 4 world Empires (in relation to Israel) from the Babylonian Empire of Daniel’s day through to the Kingdom of God.
In Daniel 7 more information is given and here the Empires take the form of animals. The fourth beast is not named but has 10 horns. History leaves little doubt but that it was the Roman Empire so depicted.
When we come to Revelation the beast with the ten horns appears again. Now it has seven heads (which I will not deal with here) but its identification is confirmed as the Roman Empire by John saying that in his day the sixth head was in power.
Revelation 17:10 "There are also seven kings. Five have fallen, one is, and the other has not yet come. And when he comes, he must continue a short time.
By this we understand that the Empire has 7 consecutive forms or seats of government. In John’s day the Empire was ruled by a pagan emperor at Rome. By the end of the 4th century the Empire was ruled by a ‘Christian’ emperor at Constantinople.
The State and the Church were combined. The Church as instituted by the apostles did not become a ‘state church’ without major changes in doctrine. Doctrines such as the trinity, division into clergy and laity, the imposition of a priesthood, immortality of the soul, heaven going at death, eternal torment in hell, ‘just’ wars etc replaced the doctrine of Christ with the commandments of men. The prophecy is intended to warn true Christians that compromise with this State Church is not possible.
Revelation 14:9 Then a third angel followed them, saying with a loud voice, "If anyone worships the beast and his image, and receives his mark on his forehead or on his hand, 10 "he himself shall also drink of the wine of the wrath of God, which is poured out full strength into the cup of His indignation.
The connection for the image is in Daniel. Nebuchadnezzar having had his dream explained, sets about building an image that everyone is to worship.
Daniel 3:1 Nebuchadnezzar the king made an image of gold, whose height was sixty cubits and its width six cubits... (Notice the beginnings of the 666 in the 60 and 6)... 4 Then a herald cried aloud: "To you it is commanded, O peoples, nations, and languages, 5 "that... you shall fall down and worship the gold image that King Nebuchadnezzar has set up;
What had happened to the Jews at Babylon happened to the Christians in the Roman Empire. The image of the beast – the State Church is set up for worship. The magnificent churches which were built show the lust of the Church for wealth and power. This gives us a second connection for the 666.
AV 1 Kings 10:14 Now the weight of gold that came to Solomon in one year was six hundred threescore and six talents of gold,
1 Timothy 6:10 For the love of money is a root of all kinds of evil, for which some have strayed from the faith in their greediness, and pierced themselves through with many sorrows.
AV Revelation 13:17 And that no man might buy or sell,
Revelation being a book of symbol we conclude that the buying and selling is not restricted to commerce but to preaching and hearing the word of God.
Isaiah 55:1 "Ho! Everyone who thirsts, Come to the waters; And you who have no money, Come, buy and eat. Yes, come, buy wine and milk Without money and without price.
To preach the word of God, official licence and adherence to corrupt doctrine was necessary. To listen to unlicensed preachers was an offence. The mark, the name, the number are all symbols for approval; the hand and the forehead are symbols for works and thoughts brought into submission.
The number six is associated with man. He was created on the sixth day. His dominion lasts for six thousand years. His measures – a cubit is six hand breadths – a reed is six cubits. This is as much as I can give you without speculation. The basic meaning of the prophecy is that many will be drawn away from the truth in Christ and become part of a false system of worship devised by man.
Colossians 2:8 Beware lest anyone cheat you through philosophy and empty deceit, according to the tradition of men, according to the basic principles of the world, and not according to Christ.
I hope this helps.