Question: "What is the unforgivable Sin and how do people commit it?"
Fr. Michael Pomazansky addresses this question very thoroughly in the context of his discussion of the sacrament of confession in his Orthodox Dogmatic Theology:
"Holy Scripture speaks of cases or conditions when sins are not forgiven. In the word of God there is mention of the blasphemy against the Holy Spirit, which "shall not be forgiven unto men, neither in this world, neither in the world to come" (Matt. 12:31-32). Likewise, it speaks of the sin unto death, for the forgiveness of which it is not commanded even to pray (1 John 5:16). Finally, the Apostle Paul instructs that "it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Spirit, and have tasted the good word of God, and the powers of the world to come, if they shall fall away, to renew them again unto repentance, seeing they crucify to themselves the Son of God afresh, and put Him to an open shame" (Heb. 6:4-6).
In all these cases, the reason why the forgiveness of sins is not possible is to be found in the sinners themselves, and not in the will of God; more precisely, it lies in the lack of repentance of the sinners. How can a sin be forgiven by the grace of the Holy Spirit, when blasphemy is spewed forth against this very grace? But one must believe that, even in these sins, the sinners, if they offer sincere repentance and weep over their sins, will be forgiven. "For," says St. John Chrysostom about the blasphemy against the Holy Spirit, "even this guilt will be remitted to those who repent. Many of those who have spewed forth blasphemies against the Spirit have subsequently come to believe, and everything was remitted to them" (Homilies on the Gospel of Matthew [Homily 41]). Further, the Fathers of the Seventh Ecumenical Council speak of the possibility of forgiveness for deadly sins: "The sin unto death is when certain ones, after sinning, do not correct themselves . . . In such ones the Lord Jesus does not abide, unless they humble themselves and recover from their fall into sin. It is fitting for them once more to approach God and with contrite heart to ask for the remission of this sin and forgiveness, and not to become vainglorious over an unrighteous deed. For 'the Lord is nigh unto them that are of a contrite heart'" (Ps. 33:18)" (Orthodox Dogmatic Theology, trans. Fr. Serpahim (Rose), (Platina, CA: St. Herman Press, 1984), p. 289f).St. Augustine makes the identical point in his sermon on Matthew 12:32. He says that it is not merely the act of blaspheming the Holy Spirit which is unpardonable, but refusing to repent of that blasphemy: "...even this shall be forgiven, if a right repentance follow it...But that blasphemy of the Spirit Himself, whereby in an impenitent heart resistance is made to this so great gift of God even to the end of this present life, shall not be forgiven" (Sermon 21 on the New Testament). So as long as a man lives, repentance is possible, but if he persists in rejecting the Holy Spirit, it is impossible for him to be pardoned, because there is no pardon without repentance.