Here is the passage from Matthew 12 in context:
"Then they brought to him a demoniac who was blind and mute; and he cured him, so that the one who had been mute could speak and see. All the crowds were amazed and said, “Can this be the Son of David?” But when the Pharisees heard it, they said, “It is only by Beelzebul, the ruler of the demons, that this fellow casts out the demons.” He knew what they were thinking and said to them, “Every kingdom divided against itself is laid waste, and no city or house divided against itself will stand. If Satan casts out Satan, he is divided against himself; how then will his kingdom stand? If I cast out demons by Beelzebul, by whom do your own exorcists cast them out? Therefore they will be your judges. But if it is by the Spirit of God that I cast out demons, then the kingdom of God has come to you. Or how can one enter a strong man’s house and plunder his property, without first tying up the strong man? Then indeed the house can be plundered. Whoever is not with me is against me, and whoever does not gather with me scatters. Therefore I tell you, people will be forgiven for every sin and blasphemy, but blasphemy against the Spirit will not be forgiven. Whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."
The Pharisees here who sought to accuse Jesus did not commit the unpardonable sin by blaspheming against Christ by accusing Him of casting out demons with the power of demons; rather they were endangering themselves by denying the redemptive work of God and the Spirit's power exercised through Christ. Not that they had already committed it, but that if they continued to be so obstinate to the reality of God's work right in front of them then their hearts were continually hardened and would oppose the work of God every step of the way.
The general consensus among most Christians is this: Blasphemy against the Holy Spirit isn't a sin one just "does" but involves a lifelong opposition against God and the Spirit's power--it is the Spirit who convicts and converts the hearts of men. Therefore the unpardonable sin is not unpardonable because God is unwilling or unable to pardon it, it is unpardonable because man refuses to be pardoned. What does this mean?
It means that if you are concerned that you have committed the unpardonable sin, then you haven't committed the unpardonable sin.
The only way you can commit such a sin is through wholesale, lifelong rejection of God's redemptive work. And, again, it's not that it is not forgiven because God refuses to forgive it--God will refuse no one who repents--it is because they will, both in this life and in the age to come refuse God's forgiveness. So that even should they, on the Day of Judgment, be offered free pardon and life everlasting they would still refuse it.
-CryptoLutheran