It's really quite simple

With the coming of the pentecostal movement, which developed into many denominations—‘he who does not gather with Me, scatters’ (Matthew 12:30b)—and its second wave, the charismatic movement, which infected almost every Christian community, causing further splits, many biblically orthodox beliefs were eroded and gradually erased from the active memory of the average member of the Church. One of these was the meaning of ‘the sin against the Holy Spirit, which cannot be forgiven’ (Matthew 12:31). Odd, how this verse follows exactly on the heels of the previous one, demonstrating they are connected. Let's quote the entire passage for the reader:

Whoever is not with me is against me, and whoever does not gather with me scatters. Therefore I tell you, every sin and blasphemy will be forgiven people, but the blasphemy against the Spirit will not be forgiven. And 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.
Matthew 12:30-32

During much of my adult life, encountering all shades of Christian believers toting their doctrinal handbags and sometimes waving them around like dangerous weapons, along with others I became somewhat confused about this ‘sin against the Holy Spirit,’ thinking that it meant being critical of what the Holy Spirit was doing in the present day, as represented by any of these ‘Spirit-filled’ communities. Nobody came right out and said it, I don't think, officially, though I did hear it on ‘Christian’ television. Finally, after many years, seeing the damage, some of it seeming beyond cure, caused by this mistaken belief originating in the Azusa Street Revival, I rediscovered the meaning of this ‘unforgivable sin.’

Rather than explain it in my own words, I want to quote it from the blog of brother Timotheos, the young choir director at our Greek Orthodox parish, Aghia Epiphania, in Jakarta, Indonesia.

How do you sin against the Holy Spirit?

There is only one way, that is by consciously refusing to accept God's forgiveness. God does not forgive because someone refuses to accept His forgiveness. You cannot do that by accident. It takes a conscious and continuous effort on your part.

“The correct interpretation, as it is given to us by the Church Fathers, is this: blasphemy against the Holy Spirit is the denial by man out of hatred of God's power to save him. Even more simply, the man who does not believe that the grace of God—the Father, the Son, and the Holy Spirit—can save him, closes his heart to the actions of the Holy Spirit; he does not accept Grace. He does not proceed to repentance. He fights against the sanctifying and saving act of God. He creates within himself a sorrowful and incurable condition.”

As you can see, it's really quite simple.