The Scriptures tell us that the Lord is willing to forgive us when we confess and forsake our sin. However, there is one category of sin that will never be forgiven and Luke 12:10 spells it out. "And anyone who speaks a word against the Son of Man (Jesus), it will be forgiven him; but to him who blasphemes against the Holy Spirit, it will not be forgiven." Compare Mark 3:29.
In its essence, "blasphemy against the Holy Spirit" involves a rejection of God the Holy Spirit, and His ministry. Remember that He is the Agent of Salvation. In other words, He is the One who causes us to be "born again". John 3:5 speaks of the necessity of being "born of water and the Spirit". Anyone who rejects the ministry of the Holy Spirit cannot, therefore, find forgiveness and peace with God. That is what makes it an unpardonable sin.
Can a believer find himself in this condition? Heb. 6:1-6 seems to indicate that it could be possible for someone who has been "enlightened" to become an apostate (defector) and fall away. Again, this will be a result of finally rejecting the Holy Spirit's Voice and refusing to turn back to the Lord. How will you know if you have committed the unpardonable sin? Ask yourself: Have I deliberately rejected Christ (in my heart and not just in my mind)? Is the Holy Spirit no longer convicting me of sin? It's very unlikely that someone in that condition will even be interested in whether or not sin is unpardonable! The very fact that you are asking the question is an indication that God is not finished with you yet!
Our God is very merciful, kind and gracious. Ps 130:3, 4 exults in that truth: "If you, Lord, should mark iniquities ... who could stand? But there is forgiveness with You, that you may be feared (revered)."