Here is what I believe to be the blasphemy against the Holy Spirit.
First lets look at the verses in context with the chapter:

 Matthew  12
  22. 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.
  23  And all the people were amazed, and said, Is not this the
 son of David?
  24  But when the Pharisees heard [it], they said, This [fellow]
 doth not cast out devils, but by Beelzebub the prince of the
 devils.
  25  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:
  26  And if Satan cast out Satan, he is divided against himself;
 how shall then his kingdom stand?
  27  And if I by Beelzebub cast out devils, by whom do your
 children cast [them] out? therefore they shall be your judges.
  28  But if I cast out devils by the Spirit of God, then the
 kingdom of God is come unto you.
  29  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.
  30  He that is not with me is against me; and he that gathereth
 not with me scattereth abroad.
  31  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.
  32  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.
  33  Either make the tree good, and his fruit good; or else make
 the tree corrupt, and his fruit corrupt: for the tree is known by
 [his] fruit.
  34  O generation of vipers, how can ye, being evil, speak good
 things? for out of the abundance of the heart the mouth speaketh.
  35  A good man out of the good treasure of the heart bringeth
 forth good things: and an evil man out of the evil treasure
 bringeth forth evil things.
  36  But I say unto you, That every idle word that men shall
 speak, they shall give account thereof in the day of judgment.
  37  For by thy words thou shalt be justified, and by thy words
 thou shalt be condemned.

  Verse 22 starts out with a man being healed by Jesus.  A miracle
that no one could deny.  In fact Jesus had done many miracles that
none could deny and this was just one more of them.
  Verse 23 has the people amazed at this miracle.  They knew and
could easily see that a great miracle had occurred.

   By doing these miracles Jesus had given irrefutable proof of who
He was and that He was from God.  People had come face to face with
the fact and had to react and make a decision.

    Verse 24 shows the religious leaders reaction.  They had already
rejected Jesus and no amount of proof would convince them.  They had
made up their minds despite the facts.  Lots of people out there
today have the same attitude.  Rather than believing the miracle
they accused Him of doing these things by the power of Satan.
Remember they made this accusation in the light of irrefutable proof
which they would not receive.

    In response, Jesus refuted the religious leaders and gave the
warning about blasphemy against the Holy Spirit in verse 31,32.

   In light of this, here is what I believe concerning the blasphemy
against the Holy Spirit:
    - Jesus must be physically in the world
    - there must be irrefutable proof as to who He is (signs,
      miracles etc)
  At that point the blasphemy against the Holy Spirit may be
committed.  Since Jesus is not physically in the world today I do
not believe the blasphemy against the Holy Spirit can be committed
in these days.  It can be committed during the 1000 year reign of
Christ after He returns...but not now.

   A Spirit-reborn Christian cannot commit the blasphemy against the
Holy Spirit because they are SEALED with the Holy Spirit.
   The closest that a person can come to blaspheming against the
Holy Spirit is to die rejecting God's salvation.  By doing that the
person is snubbing Gods free love gift of salvation and saying: I
don't want your salvation....I'll pay for my sins myself.

 Hebrews   2
  3  How shall we escape, if we neglect so great salvation; which
 at the first began to be spoken by the Lord, and was confirmed
 unto us by them that heard [him];

Alan Yusko
The Seed Sowers BBS (416) 498-5259/5962