According to the Bible, the one sin that cannot be forgiven by God, is blasphemy against the Holy Spirit. You can find this in Matthew, Mark, and Luke, but it is more definitively spelled out in Matthew 12:22-37.
I agree with Toad regarding opinions and elbows.
"So often times it happens that we live our lives in chains and we never even know we have the key."
- The Eagles
"We call them dumb animals, and so they are, for they cannot tell us how they feel, but they do not suffer less because they have no words."
- Anna Sewell