When we think about the unforgiveable sin, most recall Jesus' warning against blasphemy against the Holy Ghost (Matthew 12:31 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.). Many have attempted to define what this particular sin involves so as toe avoid its awesome consequences, but there is no real consensus.
In considering this issue of "unforgiveableness", I am drawn to another of Jesus' warnings in Matthew 6:14-15- For if ye forgive men their trespasses, your heavenly Father will also forgive you: But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.
Is there more than one unforgiveable sin? The wording of Matthew 12:31 makes that seem unlikely. Could we conclude therefore that the failure to forgive is, in fact, the one unforgiveable sin, the ultimate rejection of the very spirit and nature of God? That makes perfect sense to me. The inability or unwillingness to forgive others affects our ability to forgive ourselves because as we judge others we automatically pass judgment upon ourselves (Matthew 7:2 For with what judgment ye judge, ye shall be judged: and with what measure ye mete, it shall be measured to you again). As in all things, at the bottom of the biblical message, we find that we are not judged and condemned by God, per se. Rather we are condemned by our own self righteousness and judgment of others, all arising from the forbidden Tree of Knowledge.
There's another possible twist on the idea of the unpardonable sin. In a human sense, the unpardonable sin could be the sin of forgiving the unforgiveable, being too forgiving. That is the shortcoming of Jesus in the eyes of many. He is viewed as foolishly and naively meek, submissive, and tolerant of the mistakes of others. In other words, he seems a weakling unable or unwilling to stand up and defend himself. Such attributes are unpardonable to the human ego, trained as it is in detecting and punishing sins in others.