My understanding is that "blasphemy against the holy spirit" would be for someone who knows the truth and knows that the spirit has borne witness to them of the truth, to then lie about what they know and deny God. I don't believe that it is possible for anyone to commit that sin who doesn't know that God is real, or who hasn't received a personal witness from the spirit. It is not a sin that can be committed by mistake or unknowingly. It is a choice that very few people would make, especially among those who are in such a position as to be capable of committing that sin.