A friend once asked what was the unpardonable sin? My initial reply was, "rejecting Jesus as your Savior," but that is not what the Bible says.
In Matthew 12:31-32, Jesus says, "Therefore I say to you, any sin and blasphemy shall be forgiven men, but blasphemy against the Spirit shall not be forgiven. And whoever shall speak a word against the Son of Man, it shall be forgiven him; but whoever shall speak against the Holy Spirit, it shall not be forgiven him, either in this age, or in the age to come."
The unpardonable sin is blasphemy against the Holy Spirit. Blasphemy is any act of effrontery by man that dishonors and insults the holiness, character, or actions of God. The sin of blasphemy is a deliberate rejection of that which man knows to be of God. That is why the Bible says in Romans 1 and 2 God's nature and attributes are clearly visible to man and man rejects God (that is the exercise of a spiritually dead sinner's free will), and God's judgement falls on man because man is without excuse.
The unpardonable sin is not man saying to the Holy Spirit when He gives the sinner new life and calls him to follow Jesus, "I want to exercise my free will and say, 'No.'" No, the regenerated man who hears that call will ultimately follow Jesus. The unpardonable sin is fallen, un-regenerated, spiritually-dead man suppressing the truth in unrighteousness, and denying the invisible attributes, eternal power, and divine nature of God that have been clearly seen and known since creation.