There are two references that are associated with what people have referred to as the unpardonable sin.
The reference in Matthew 12:31,32 where “Blasphemy against the holy Ghost shall not be forgiven…in this world or the world to come” is considered unpardonable. And this is true in the Kingdom program then and in the Kingdom to come, but not in the age of grace.
Also, in I John 5:16 there is “a sin unto death” that if a man commits, no one should even pray for him. In the context of the coming anti-Christ (I John 2:18; 4:3) this would be a reference to someone who takes the mark of the beast and is then damned. Again, this sin cannot be committed in the age of grace.
The third program on our Forgotten Truths Volume 21 is about “The Unpardonable Sin.” You may have that volume. If you would like the transcript of that program let me know and I can send it to you.