Luke 12:8-12 (ESV)
"And I tell you, everyone who acknowledges me before men, the Son of Man also will acknowledge before the angels of God,  but the one who denies me before men will be denied before the angels of God.  And everyone who speaks a word against the Son of Man will be forgiven, but the one who blasphemes against the Holy Spirit will not be forgiven.  And when they bring you before the synagogues and the rulers and the authorities, do not be anxious about how you should defend yourself or what you should say,  for the Holy Spirit will teach you in that very hour what you ought to say."
The sin against the Holy Spirit, blaspheming the Holy Spirit. It won’t be forgiven, but only those who haven’t committed it are worried about it. It seems that makes sense. Non Christians certainly don’t worry whether or not they have blasphemed the Holy Spirit. Though much of what passes for Christian doctrine seems to come close to blaspheming the Holy Spirit. The only thing there is I have seen Baptists convert and be forgiven. Then again, maybe the exception proves the rule. It’s serious stuff people. Denying the grace of God to those Jesus cares for so much, simply because, despite the entire New Testament, you can’t bring yourself to believe the Holy Spirit can work through baptism is despicable.
But Jesus says this as a warning for those about to be tortured and tried in his name. don’t worry, the Holy Spirit will tell you what to say. Say that. It’s true. It is true even in personal evangelism with friends. Acknowledging Jesus there, will be rewarded by Jesus on the last day. The Holy Spirit will guide you in what you say. In the end it is all his work.