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Blasphemy is the act of treating with contempt, cursing or injuring the good name of another - especially God.
The Jews accused Christ of blasphemy when he forgave sins, believing that he was making himself God by forgiving sins. The reasoning is that anyone who makes himself God demeans God.
Matthew 9: 2 Then behold, they brought to Him a paralytic lying on a bed. When Jesus saw their faith, He said to the paralytic, "Son, be of good cheer; your sins are forgiven you." 3 And at once some of the scribes said within themselves, "This Man blasphemes!"
It was for the perceived blasphemy of saying that he was the son of God that they condemned Jesus to death.
Matthew 26:65 Then the high priest tore his clothes, saying, "He has spoken blasphemy! What further need do we have of witnesses? Look, now you have heard His blasphemy!
The only blasphemy that will not be forgiven is that spoken against the Holy Spirit.
Mark 3:29 "but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation" — 30 because they said, "He has an unclean spirit."
Paul is an example of a blasphemer forgiven.
1 Timothy 1:13 although I was formerly a blasphemer, a persecutor, and an insolent man; but I obtained mercy because I did it ignorantly in unbelief.
I hope this helps.