There has been much talk and speculation about this “sin” down through the centuries. Certainly there has been much undue worry about whether “I’ve committed the unforgivable sin” in the minds of many people. There’s even a web site dedicated to trying to commit the unforgivable sin.
Anyhow, it seems to me that the only sin that is unforgivable is the sin that denies the need for forgiveness, or denies there to be a means of forgiveness. It seems to me that denying the existence of God would cover both of those scenarios.
“Glory to God in the highest and peace on earth to all men who please Him”
There is no peace for those who will not give glory to God in the highest.