Answer: The word blasphemy is mentioned fourteen times in the King James Bible. It is used only twice in the Old Testament and twelve times in the New. Two of the main Scriptures that use the word are found in the books of Matthew and Revelation.
In Matthew, Christ uses the word blasphemy to warn the Pharisees who accuse him of using the power of the devil to cast out demons. In Revelation, the word is used in reference to the Beast power that will soon take control of the entire world and force the worship of Satan onto all people.
Because of this, I say to you, every sin and blasphemy shall be forgiven to men except the blasphemy against the Holy Spirit; that shall not be forgiven to men.
And whoever speaks a word against the Son of man, it shall be forgiven him; but whoever speaks against the Holy Spirit, it shall not be forgiven him, neither in this age nor in the coming age (Matthew 12:31 - 32, HBFV throughout).
And I stood on the sand of the sea; and I saw a beast rising up out of the sea, having seven heads and ten horns, and on his horns ten crowns, and upon his heads the name of blasphemy (Revelation 13:1, see also 13:6 and 17:3).
The English word in question is translated, in all twelve of its New Testament references, from the Greek word blasphemia (Strong's Concordance #G988). It means to vilify, speak evil of, or rail against a person. Webster's Dictionary defines the word as to insult, show contempt of or not revere God sufficiently. The word can be applied to a person who claims some or all of God's attributes. The word "profane," a synonym, is listed as abusing or treating with contempt or disdains that which is sacred.
Not an accident
The above shows that blasphemy is not done by accident. It is done when one has the knowledge of the person, object, or idea that is being discussed. This was the initial charge against Jesus, made by Jewish religious leaders, that justified he be put to death (Matthew 26:63 - 66).
The Bible makes a distinction between what it means to commit this act against the Son of Man (Jesus) versus the Holy Spirit. This, however, begs the question as to why there is a distinction.
Jesus versus Holy Spirit
Why are sins against Jesus forgiven but not those against the Holy Spirit? Does this mean that the Spirit, given upon repentance and baptism, is somehow greater than the Savior of mankind who made the gift possible?
People, out of their lack of full knowledge and understanding, may wrongly commit blasphemy against Jesus and still receive forgiveness.
For example, many people living at the time of Christ, who saw his miracles and heard him speak, concluded that he was from God. However, they also blasphemed because they denied and rejected He was God's son and the Messiah. Yet, because of their ignorance, this sin (upon repentance) will be forgiven.
In fact, a person's rejection of Jesus and his message can sometimes be due to God himself hardening their hearts. Why would He do such a thing?
. . . so that they would not see with their eyes and understand with their hearts, and be converted, and I would heal them (John 12:40).
Speaking against or committing blasphemy against the Spirit is a different matter entirely. Although the Eternal certainly worked with a few people in the Old Testament, he did not make His Spirit available to mankind in general until Pentecost in 30 A.D. (Acts 2:16 - 21).
To receive of the gift of God's Spirit, a person must accept and believe that Jesus is God's son and the Savior of the world, repent of sin, be willing to surrender their entire life to His will, and go under the waters of baptism (Acts 2:38).
A sin that is unpardonable
If, after all of the above and receiving the spirit a person, with full knowledge, rejects God and commits blasphemy (has contempt for the spirit within them), her or she then becomes one of His adversaries (like the devil). Someone who does this has no hope of forgiveness, as they now refuse to repent of their sins and change. The apostle Paul explains this further with the following.
For there is no longer any sacrifice that will take away sins if we purposely go on sinning after the truth has been made known to us (Hebrews 10:26).
The God of the Bible is one of mercy, forgiveness, and love. However, if a person willfully chooses to commit blasphemy against him, he will honor their choice. He will ultimately put them out of their misery in the lake of fire (Revelation 20:14 - 15).