Matthew 12:31 says that every sort of sin and blasphemy will be forgiven, but blasphemy against the holy spirit will not be forgiven. Willful and deliberate opposition to the operation of God's holy spirit is blasphemy against the holy spirit. An example is the pharisees in Jesus' time, who although they saw the miracles and good works he performed, attributed his power to the Devil.
Outline of Biblical Usage: 1) to speak reproachfully, rail at, revile, calumniate, blaspheme 2) to be evil spoken of, reviled, railed at
Blasphemy against The Holy Ghost is to speak evil of it; to belittle or take away from it. For instance, when Jesus addressed this issue in (KJV) Mark 3:29 But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation:(KJV) Mark 3:30 Because they said, He hath an unclean spirit
You see they attributed The power of God to the devil... Saying that Jesus had Cast Out demons by a demonic power. Jesus began to warn them about blasphemy.
Giving The Glory for God's power to some other source is a form of blasphemy. Mockery, etc.
Be very cautious: no forgiveness is a very serious place to Find oneself in