Jesus mentions a sin that is unforgivable in Matt. 12:31-32 and calls it blasphemy of the Holy Spirit. But what exactly is this unforgivable sin?
Matt. 12:22-32 says, "Then there was brought to Him a demon-possessed man who was blind and dumb, and He healed him, so that the dumb man spoke and saw. 23And all the multitudes were amazed, and began to say, "This man cannot be the Son of David, can he?" 24But when the Pharisees heard it, they said, "This man casts out demons only by Beelzebub the ruler of the demons." 25And knowing their thoughts He said to them, "Any kingdom divided against itself is laid waste; and any city or house divided against itself shall not stand. 26"And if Satan casts out Satan, he is divided against himself; how then shall his kingdom stand? 27"And if I by Beelzebub cast out demons, by whom do your sons cast them out? Consequently they shall be your judges. 28"But if I cast out demons by the Spirit of God, then the kingdom of God has come upon you. 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. 30"He who is not with Me is against Me; and he who does not gather with Me scatters. 31"Therefore I say to you, any sin and blasphemy shall be forgiven men, but blasphemy against the Spirit shall not be forgiven. 32"And whoever shall speak a word against the Son of Man, it shall be forgiven him; but whoever shall speak against the Holy Spirit, it shall not be forgiven him, either in this age, or in the age to come," (All Scripture quotes are from the NASB).
Jesus did His miracles by the power of the Holy Spirit (rather than out of his divinity) who had come upon Him at His baptism and had lead him into and out of the wilderness temptings. The Pharisees were a prominent group of religious leaders who held strong political influence in the Jewish community of the first century. The knew that Jesus' miracles validated His teaching and ministry (see John 11:45-48) and they were attempting to discredit Jesus' ministry (and thereby his role as Messiah) by saying that His works were by the devil and not by the Holy Spirit. Therefore, when the Pharisees accused Jesus of casting out demons by the power of Satan, they were blaspheming (slandering) the Holy Spirit by whom Jesus performed His miracles. This is unforgivable because they knowingly sought to slander what God was doing in order to protect their own influence. In so doing, they were alienating further those who so desperately needed salvation.
There is no biblical support for a believer committing this sin. If you are worried that you may have committed the sin and can’t be forgiven, then don’t be concerned. If you are worried about it, then that is a sign that you have not committed it. If you had, you probably wouldn’t be concerned about it.