It's not adultery. Jesus said to the woman committing adultery and caught in the very act: "Neither do I condemn thee; go and sin no more."
It isn't theft. He said to Zaccheus, "This day is salvation come upon thy house." Zaccheus had been a thief.
It's not murder. Men's hands have been red with blood and God has forgiven them. The Apostle Paul's hands were red with blood.
What is it? To me it is plain and simple. It is constant and continual, and final rejection of Jesus Christ as your Saviour. God's offer of mercy and salvation comes to you and you say, "No," and you push it aside. I do know that there is such a thing as the last call to every man or woman. God says that his spirit will not always strive with man, and when a man or woman says "No" as God's spirit strives for the last time it forever seals your doom.
It is no special form of sin, no one act. It might be swearing, it might be theft. Any one becomes unpardonable if God keeps calling on you to forsake that sin and you keep on refusing to forsake it, and if you don't then he will withdraw and let you alone and that sin will become unpardonable, for God won't ask you again to forsake it.
It is no one glaring act, but the constant repetition of the same thing. There will come a time when you commit that sin once too often.
It is a known law of mind that truth resisted loses its power on the mind that resists it. You hear a truth the first time and reject it. The next time the truth won't seem so strong and will be easier to resist. God throws a truth in your face. You reject it. He throws again; you reject again. Finally God will stop throwing the truth at you and you will have committed the unpardonable sin.
"There is a line by us unseen;
It crosses every path;
It is God's boundary between