Actually there is sin that cannot be fogiven. Blasphemy against the Holy Spirit is an unforgivable sin according to Our Lord and Saviour Jesus Christ.
What does "blaspheming the Holy Spirit" mean? I've always been taught that it had to do with a despondency that made one feel that no sin he or she committed was forgiveable, and, hence, did not allow the Holy Spirit to work in one's life and forgive. Basically, an unwillingness to turn to God because of a feeling that not even God Himself could fix things, which is so incredibly unthinkable and, hence, a blasphemy.