Jesus mentions a sin that is unforgivable in Matt. 12:31-32 and calls it blasphemy of the Holy Spirit. What exactly is this unforgivable sin? For that, we need to look at the context.
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).
Let me review this section briefly. In verse 22, Jesus healed a blind and dumb man. The Pharisees accuse Jesus of casting out demons by the power of "Beelzebub, the ruler of the demons" (v. 24). Jesus responds by saying that a kingdom divided will fall (vv. 25-28), and how the devil must first be bound before you can plunder his house (v. 29). In verses 31-32, He states that blasphemy against the Spirit will not be forgiven in this age or the age to come.
By simply looking at the context it becomes apparent that blasphemy of the Holy Spirit is saying that Jesus did His miracles by the power of the devil. This is unforgivable. But why? We can find a clue by looking at the point when Jesus began His ministry.
Jesus stated that His baptism was to "fulfill all righteousness," (Matt. 3:15). The word "fulfill" should cause us to think of the Old Testament. Basically, Jesus was baptized because He had to fulfill the Old Testament requirements for entering into the priesthood. He was a priest after the order of Melchizedek (Psalm 110:4; Heb. 5:8-10; 6:20). Priests offered sacrifice to God on behalf of the people. Jesus became a sacrifice for our sin (1 Pet. 2:21; 2 Cor. 5:21) in His role as priest. According to the Old Testament, in order for a priest to be consecrated, he had to be washed with water (Lev. 8:6; Exodus 29:4, Matt. 3:13-15) and anointed with oil (Lev. 8:12; Exodus 29:7; Matt. 3:16). Both of these were bestowed upon Jesus at His baptism. Additionally, He may have needed to be 30 years old (Num. 4:3).
The oil is representative of the Holy Spirit who descended upon Jesus at His baptism (Matt. 3:16). It was after His baptism that He began His ministry and started performing miracles. He did His miracles by the power of the Holy Spirit who had come upon Him at His baptism. The Pharisees - who knew that Jesus' miracles validated His words and ministry (see John 11:45-48) - were attempting to discredit Jesus' Messiahship by saying 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 the Holy Spirit by whom Jesus performed His miracles. This is unforgivable because it struck at the very heart of the redemptive work of God in Christ. It struck at the very nature of Jesus' ministry of redemption, testimony, and teaching. Jesus was ministering in the power of the Holy Spirit Himself, fulfilling the divine plan of God to provide a sacrifice for our sins (John 3:16; 1 John 4:10). The Pharisees were attributing this to demonic activity. This is a great blasphemy.
Can a Believer Commit the Unforgivable Sin?
No. A believer cannot commit the unforgivable sin. How can someone who has been born again (John 3:7), made a new creation (2 Cor. 5:17), and received eternal life (John 10:27-28) actually commit the unforgivable sin? He cannot. Jesus Himself said that we have eternal life, not conditional life: "My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they shall never perish; and no one shall snatch them out of My hand" (John 10:27-28). Besides, it says in 2 Cor. 5:17 that the Christian is a new creation in Christ. We are different, no longer slaves to the old nature (Rom. 6:14). We are regenerated by the Holy Spirit.
There is no biblical support for a believer committing this sin. It just hasn't happened. Also, if you are worried that you may have committed the sin and can't be forgiven, then don't be concerned. If you are worrying about it, then you haven't committed it. If you are worried about it, that is a sign you have not committed it. If you had, you wouldn't be concerned.