First, let’s look at the scripture. The passage that talks about this topic is found in Mathew 12:31-32 “And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.”
In this passage, Jesus is addressing a group of Pharisees who, after seeing Jesus heal a possessed man, accuse Jesus of using Beelzebul to cast out demons. Beelzebul another name for Satan and he is a very evil power, so Jesus did not take this accusation lightly. I talk more about this topic in my study about demon possessions.