The unforgivable sin
What the blasphemy against the Spirit is, and whether it can be committed today.
Matthew 12:31-32
31Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men.
32Whoever speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, neither in this age, nor in that which is to come.
What each tradition says
Rabbinic / Midrashic
Jewish theology does not admit that there is an unpardonable sin. The Mishnah holds that sins are expiated by sacrifice, by repentance at death or on Yom Kippur, and, in the case of lighter transgressions of the positive or negative precepts, by repentance at any time.
The Jewish Encyclopedia (1905), The Jewish Encyclopedia (1905), art. "Sin" — Section on the unpardonable sin
Patristic
Augustine says that when one stands as an enemy to this gift, not seeking it in repentance but opposing it by impenitence, his sin becomes unpardonable. It is not any one specific kind of sin, but contempt for or opposition to the remission of sins itself.
Catholic
Aquinas says that if the sin against the Holy Ghost is understood as final impenitence, it is called unpardonable. Such a person perseveres in mortal sin until death, so it is not forgiven in the life to come, because it was not remitted by repentance in this life.
Thomas Aquinas, Summa Theologica (Fathers of the English Dominican Province trans., 2nd revised ed.) — II-II q.14 a.3 — Whether the sin against the Holy Ghost can be forgiven?
Orthodox
Chrysostom says that these sins are forgiven on repentance, and no penalty is exacted. But blasphemy against the Spirit shall not be forgiven, not even to those who repent.
John Chrysostom, Homilies on the Gospel of Saint Matthew (Nicene and Post-Nicene Fathers, First Series, Vol. 10) — Homily XLI, on Matthew 12:31-32
Reformed / Protestant
Calvin concludes that those who maliciously turn the perfections of God to the Spirit’s dishonor—perfections revealed to them by the Spirit and in which God’s glory ought to be celebrated—sin and blaspheme against the Holy Spirit. With Satan as their leader, they are avowed enemies of God’s glory.
John Calvin, Commentary on Matthew, Mark, Luke — Volume 2 (Calvin Translation Society) — On Matthew 12:31-32
Evangelical
Wesley says whoever speaks thus against the Holy Ghost will not be forgiven, neither in this world nor the world to come. The phrase was a Jewish proverb for something that would never be done, and here means the offender will not escape punishment, in this world or the next. God's judgment will overtake him both here and hereafter.
John Wesley, Wesley's Notes on the Bible (Notes on the Gospel According to St Matthew) — Note on Matthew 12:31-32