Blasphemy of the Holy Spirit is:
You must trust in Christ as your Savior to have your sins forgiven, and your sins must be forgiven if you want to go to heaven when you die.
Everyone sins, even those who consider themselves to be a good person (Take the Good Person Test). Being a good person in the world does not mean you are not a sinner; everyone sins. All sins can be forgiven with the exception of Blasphemy of the Holy Spirit.
Blasphemy of the Holy Spirit is the rejection of the Holy Spirit’s convincing you of your need for salvation through Jesus Christ. If you reject the Holy Spirit’s call to repentance, you will never trust in Christ as your Savior, your sins cannot be forgiven, and you cannot be saved.
That is because the only way for anyone to know that he or she needs Christ as Savior is for the Holy Spirit to convince him or her that they are sinners. In your flesh, you will never truly understand your need for Christ’s salvation.
Rejecting the Spirit means you will never understand the need for a Savior; the result is that you will never be saved and your sins will never be forgiven. Thus you will spend eternity in isolation from God in a very bad place that the Bible calls the lake of fire, instead of in heaven.