Matthew 12:31-32 says, "Every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come."
The backdrop to this passage is that the Jewish leaders who had just witnessed a mighty miracle of Christ should have recognized that Jesus performed this miracle in the power of the Holy Spirit. After all, the Hebrew Scriptures, of which the Jewish leaders were well familiar, prophesied that when the Messiah came He would perform many mighty miracles in the power of the Spirit (see Isaiah 35:5-6). Instead, these Jewish leaders claimed that Christ did this and other miracles in the power of the Devil, the unholy spirit. This was a sin against the Holy Spirit. This shows that these Jewish leaders had hardened themselves against the things of God.
I believe that Matthew 12 describes a unique situation among the Jews, and that the actual committing of this sin requires the presence of the Messiah on earth doing His messianic miracles. In view of this, I don't think this sin can be reduplicated exactly today as described in Matthew 12.
I think it's also important to realize that a human being can repent of his or her personal sins (whatever they be) and turn to God as long as there is breath still left in his or her lungs. Until the moment of death, every human being has the opportunity to turn to God and receive the free gift of salvation (Ephesians 2:8-9).
