According to Revelation 13:6, John tells us that the beast opened his mouth in blasphemy (G988) against God, and he blasphemed (G987) the name of God, he blasphemed his Tabernacle, and he blasphemed those in heaven. The Greek word for blaspheme, according to the Analytical Greek Lexicon, means ‘calumny’ ‘railing’ ‘reproach’ ‘blasphemy’. The word has to do with slander, a falsification or a misrepresentation intended to discredit another. When used of God it is called blasphemy when used of man it is called slander, but the meaning is the same.
Paul used the word in Ephesians 4:31, saying it was something believers ought not to do. The KJV translates the Greek into “evil speaking” while the NASB renders it “slander.” This is the basic meaning of the word. It is slandering God, that is, blaspheming his name. This could be done by misrepresenting him, i.e. for a person to say God sent him when he didn’t. It is slandering God or blaspheming him to falsify his character, which is to tell people what God is like, when what is said about God is not what he is like. This very same thing can be done against the Lord’s disciples, as well, by saying one is sent by them (cf. Galatians 2:12; Acts 15:1, 23-24; 1John 2:19), when that one was not sent. It can also be done by seeking to discredit the Lord’s disciple by saying he said something he hadn’t said (cf. Romans 3:8 and the Greek word in question is G987).
In the context of Revelation 13:6, I believe the word is used to show the beast simply discredits its enemies. He speaks in slanderous ways and does it loudly in the sense that he makes himself heard, and, because he does so, the opposition is unable or powerless to adequately oppose him. Such was the case, as it pertained to Jesus and later his disciples. They were simply unable to oppose the Jewish authorities, who were not above placing slanderous labels on their opponents (cf. John 18:30; Mark 15:3; Luke 23:5; Acts 17;6; 24:5)
The Greek word for blaspheme is blasphemeo (G987) and, besides here, it is used only in Revelation 16:9, 11 and 21 in the Apocalypse. There men blasphemed the name of God after the fourth vial or bowl was poured out (Revelation 16:9). Again they blasphemed after the fifth bowl judgment was poured out (Revelation 16:11). Finally, after the seventh vial or bowl was poured out in judgment, and as great hail stones were poured out upon men, they blasphemed God (Revelation 16:21). The point is that in an earlier study on Revelation 8:7 I showed that the hail stones were actually the Apostles and prophets that Jesus sent to preach the Gospel to the Jews, which was to be done in that generation of Jews that rejected Jesus as their Messiah (Matthew 23:34; cf. 1Peter 2:5).
Not only was the beast permitted to slander the saints, and, especially, those who preached the Gospel, but it was also given him to make war (polemos, G4171) with them and overcome them. I don’t believe the sense, here, is to actually wage a war with the Lord’s disciples in the sense that we understand war. How would one do that? A nation wages war against another nation and invades the other’s land with a view of conquering its people. Believers have no land or country of their own. We live among the nations all over the world. The Greek word in this verse also has the sense of fighting or disputing. The point is that the beast was given the authority to fight with believers, i.e. persecute them. Paul, before he became a believer, and when he was known as Saul, said he was given authority to seize and arrest believers, even in foreign lands, and bring them back to Jerusalem to be punished, perhaps even executed (Acts 9:1-2; 22:3-5; 26:9-11).
 I say this in the worldly sense. Certainly, Jesus and the Apostles did what they did in the power of God, and no one is able to oppose God. However, in the sense that God permits men to oppose him as long as that opposition wouldn’t defeat his purposes, the worldly powers are free to oppose and even overcome the messengers of God, believing they are able to gain the victory (cf. the Parable of the Vinedresser (or Wicked Tenants) – Luke 20:14-15).
 Thayer’s Greek Lexicon of New Testament Words says polemos (G4171) means: “1) a war; 2) a fight, a battle; 3) a dispute, strife, quarrel.” We get our word polemic from this Greek word.