The Gospels of Matthew, Mark, and Luke describe the unpardonable sin of blasphemy of the Holy Ghost. This is a sin that is often misunderstood because the entire context is not taken into account. We can understand what this unpardonable sin is by looking at these three Gospels.
Let's first start by reading the account from Matthew:
"Then was brought unto Him one possessed with a devil, blind, and dumb: and He healed him, insomuch that the blind and dumb both spake and saw. And all the people were amazed, and said, is not this the son of David? But when the Pharisees heard it, they said, This fellow doth not cast out devils, but by Beelzebub the prince of the devils. And Jesus knew their thoughts, and said unto them, every kingdom divided against itself is brought to desolation; and every city or house divided against itself shall not stand: and if Satan cast out Satan, he is divided against himself; how shall then his kingdom stand? And if I by Beelzebub cast out devils, by whom do your children cast them out? therefore they shall be your judges. But if I cast out devils by the Spirit of God, then the Kingdom of God is come unto you. Or else how can one enter into a strong man's house, and spoil his goods, except he first bind the strong man? and then he will spoil his house. He that is not with me is against me; and he that gathereth not with me scattereth abroad. Wherefore I say unto you, all manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of Man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come." - Matthew 12:22-32
We can learn from this passage in Matthew a few important clues. The first clue is that the Pharisees believed that Jesus' work was Satanic - that He was casting out demons in the name of Satan (v24). Also, Jesus said that there is a difference between blaspheming Him and blaspheming the Holy Spirit (v31-32). Finally, we learned that Jesus spoke of two dispensations (v32).
"And whosoever shall speak a word against the Son of Man, it shall be forgiven him: but unto him that blasphemeth against the Holy Ghost it shall not be forgiven." - Luke 12:10
Luke's account is very brief, but does reiterate that God does see a difference between blasphemy of the Son and of the Holy Ghost.
"And the scribes which came down from Jerusalem said, He hath Beelzebub, and by the prince of the devils casteth He out devils. And He called them unto Him, and said unto them in parables, How can Satan cast out Satan? And if a kingdom be divided against itself, that kingdom cannot stand. And if a house be divided against itself, that house cannot stand. And if Satan rise up against himself, and be divided, he cannot stand, but hath an end. No man can enter into a strong man's house, and spoil his goods, except he will first bind the strong man; and then he will spoil his house. Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme: But he that shall blaspheme against the Holy Ghost hath never forgiveness, but is in danger of eternal damnation. Because they said, He hath an unclean spirit." - Mark 3:22-30
By reading Mark's account we gain the final clues as to what Jesus meant when He spoke about this unforgivable sin. The key to understanding what the sin is can be found by "rightly dividing the Word of Truth" (2 Tim. 2:15).
According to the three Gospels, the Pharisees witnessed Jesus healing a demon-possessed individual (Matt. 12:22) and claimed that He not only cast out the demon by the power of Satan (Matt. 12:24), but was filled with Satan (Mk. 22, 30) and said that to His face. Jesus responded with telling the Pharisees that this sin would not be forgiven now or in the future (Matt. 12:32).
Understanding dispensations is paramount to knowing when these time periods take place.
During our current dispensation, we are saved solely through the Gospel and not of works (1 Cor. 15:1-4, Eph. 2:8-9), and we cannot lose our salvation (Jn. 3:16, 6:39; Rom. 5:1, 11:29; 2 Cor. 5:17; Titus 1:2; and 1 Peter 1:18-19).
Notice what the Apostle Paul said in Romans, "But God commendeth His love toward us, in that, while we were yet sinners, Christ died for us. Much more then, being now justified by His blood, we shall be saved from wrath through Him" (5:8-9).
Realize the importance of what Paul said: we are justified BY His blood. Once Christ died for our sins, the New Testament began (Heb. 9:15). The Pharisees accused Jesus, to His face, under the Old Testament.
There are only two dispensations that mankind has the honor of seeing Jesus: the end of the Old Testament and during the Millennial Kingdom.
By "rightly dividing" (2 Tim. 2:15) the three Gospels and by understanding that the Bible shows us that there are different dispensations, we can understand just what blasphemy of the Holy Ghost is:
It is claiming that the Holy Ghost is Satan and telling that to Jesus' face.