We are told what this is... there have been a few studies on this here over the years.
Matthew's version.
Matt 12:24; But when the Pharisees heard this, they said, "This man casts out demons only by Beelzebul the ruler of the demons."
Matt 12:25; And knowing their thoughts Jesus said to them, "Any kingdom divided against itself is laid waste; and any city or house divided against itself will not stand.
Matt 12:26; "If Satan casts out Satan, he is divided against himself; how then will his kingdom stand?
Matt 12:27; "If I by Beelzebul cast out demons, by whom do your sons cast them out? For this reason they will be your judges.
Matt 12:28; "But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you.
Matt 12:29; "Or how can anyone enter the strong man's house and carry off his property, unless he first binds the strong man? And then he will plunder his house.
Matt 12:30; "He who is not with Me is against Me; and he who does not gather with Me scatters.
Matt 12:31; "Therefore I say to you, any sin and blasphemy shall be forgiven people, but blasphemy against the Spirit shall not be forgiven.
Matt 12:32; "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, either in this age or in the age to come.
The Pharisees were accusing Jesus of doing things by the power of Satan... Jesus lets them know it was the power of God.
Luke 12:10; "And everyone who speaks a word against the Son of Man, it will be forgiven him; but he who blasphemes against the Holy Spirit, it will not be forgiven him.
Now why did Jesus even say this to the Pharisees? Mark has the answer.
Mark 3:22; The scribes who came down from Jerusalem were saying, "He is possessed by Beelzebul," and "He casts out the demons by the ruler of the demons."
Mark 3:23; And He called them to Himself and began speaking to them in parables, "How can Satan cast out Satan?
Mark 3:24; "If a kingdom is divided against itself, that kingdom cannot stand.
Mark 3:25; "If a house is divided against itself, that house will not be able to stand.
Mark 3:26; "If Satan has risen up against himself and is divided, he cannot stand, but he is finished!
Mark 3:27; "But no one can enter the strong man's house and plunder his property unless he first binds the strong man, and then he will plunder his house.
Mark 3:28; "Truly I say to you, all sins shall be forgiven the sons of men, and whatever blasphemies they utter;
Mark 3:29; but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin"—
Mark 3:30; because they were saying, "He has an unclean spirit."
Notice Mark 3:30. -- because they were saying, "He has an unclean spirit
If we say things done by the power of the Holy Spirit are being done by Satan ( casting out demons by Beelzebul for example )
then we are blaspheming the Holy Spirit.
I've seen people say speaking in tongues is of the devil.... this is very thin ice.