It is important first to look at all the passages concerning this question:
Matthew 12:31-32, “Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. And whosoever says a word against the Son of man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come” (RSV).
Mark 3:28-30, “Verily, I am saying to you that all shall be pardoned the sons of mankind, the penalties of the sins and the blasphemies, whatsoever they should be blaspheming, yet whoever should be blaspheming against the holy spirit is having no pardon for the eon, but is liable to the eonian penalty for the sin---for they said, ‘An unclean spirit has he’” (CV).
The key is in Matthew where Jesus clarifies that blasphemy against the holy Spirit will not be forgiven either in this AGE or in the AGE to come. It is not an “unforgivable sin.”
The confusion is the erroneous translation of Mark 3:29 in most versions. “Hath never forgiveness” is a poor interpretation of the original Greek. The CV above being a more literal translation reads, “...is having no pardon for the eon…”, “eon” being the transliteration of the Greek “aion,” which is “age,” as in Matthew 12:32 above.
And since the noun “aion” is “age;” then “aionian,” the adjective, would be “age-during” or “age-abiding.” The CV transliterates as “eonian” as shown above.