What does it mean to speak against the Holy Spirit?

No. It doesn't directly mean denying the existence of God is the unpardonable sin. But let me give you some commentary from Matthew Henry, who wrote commentary on the whole old testament and all the way through the gospels in the new testament before he died. What he basically states in this part of the commentary is that the unpardonable sin is the one in which we DENY something or some miraculous work/s (that could only have come from heaven even when we have seen it clearly or even felt it inside of us) by saying it is not from God, even when we have been given all of the proof that there is a holy and righteous God that rules this world, then we will have committed this sin because there will be no other way for us to come back to serve God if he has given us EVERYTHING he can to show us he is the way and we have denied it. That is why it won't be forgiven in the next world because we will have made it impossible for ourselves to recover from it in this one.

Matthew Henry Commentary Full on Matthew 12:30-32

"What is this sin; it is speaking against the Holy Ghost. See what malignity there is in tongue-sins, when the only unpardonable sin is so. But Jesus knew their thoughts, v. 25. It is not all speaking against the person or essence of the Holy Ghost, or some of his more private operations, or merely the resisting of his internal working in the sinner himself, that is here meant; for who then should be saved? It is adjudged in our law, that an act of indemnity shall always be construed in favour of that grace and clemency which is the intention of the act; and therefore the exceptions in the act are not to be extended further than needs must. The gospel is an act of indemnity; none are excepted by name, nor any by description, but those only that blaspheme the Holy Ghost; which therefore must be construed in the narrowest sense: all presuming sinners are effectually cut off by the conditions of the indemnity, faith and repentance; and therefore the other exceptions must not be stretched far: and this blasphemy is excepted, not for any defect of mercy in God or merit in Christ, but because it inevitably leaves the sinner in infidelity and impenitency.

We have reason to think that none are guilty of this sin, who believe that Christ is the Son of God, and sincerely desire to have part in his merit and mercy: and those who fear they have committed this sin, give a good sign that they have not.

The learned Dr. Whitby very well observes, that Christ speaks not of what should be (Mark iii. 28; Luke xii. 10); Whosoever shall blaspheme. As for those who blasphemed Christ when he was here upon earth, and called him a Winebibber, a De- ceiver, a Blasphemer, and the like, they had some colour of excuse, because of the meanness of his appearance, and the prejudices of the nation against him; and the proof of his divine mission was not perfected till after his ascension; and therefore, upon their repentance, they shall be pardoned: and it is hoped that they may be convinced by the pouring out of the Spirit, as many of them were, who had been his betrayers and murderers. But if, when the Holy Ghost is given, in his inward gifts of revelation, speaking with tongues, and the like, such as were the distributions of the Spirit among the apostles, if they continue to blaspheme the Spirit likewise, as an evil spirit, there is no hope of them that they will ever be brought to believe in Christ; for First, Those gifts of the Holy Ghost in the apostles were the last proof that God designed to make use of for the confirming of the gospel, and were still kept in reserve, when other methods preceded. Secondly, This was the most powerful evidence, and more apt to convince than miracles themselves. Thirdly, Those therefore who blaspheme this dispensation of the Spirit, cannot possibly be brought to believe in Christ; those who shall impute them to a collusion with Satan, as the Pharisees did the miracles, what can convince them? This is such a strong hold of infidelity as a man can never be beaten out of, and is therefore unpardonable, because hereby repentance is hid from the sinner's eyes."

it is by stubborn pride that if one refuses to ask to be forgiven ,or to accept forgiveness is against the Holy Spirit either now or after death

If it were up to me it would mean anyone who says 'there is no God'. But it actually means what Oleg said.

Take heed.

TROUBLE....GOD IS THE ONLY WAY TO SALVATION AND JOY AND EVERLASTING LIFE....SPIRIT IS THE POWER OF GOD...seek Jesus thru praying..