The Church's teaching is clear: there is no limit to God's mercy (Catechism of the Catholic Church, n. 1864). Christ died to redeem the sins of all men, and any man who repents and follows Christ can be pardoned and receive a new life. Jesus told his servant, Sister Faustina Kowalska, a Polish nun who died in 1938 and was canonized by St. John Paul II in the year 2000: "Let no soul be afraid to come to Me even if its sins are scarlett red. My mercy is such that, for all eternity, no spirit whether man or angel, can possibly fathom it."
So, why does Jesus say in the gospels that there exists one sin that will never be forgotten, not in this world nor in the next: blasphemy against the Holy Spirit? One can read in Mark 3: 28-29: « Amen I say to you, that all sins shall be forgiven unto the sons of men, and the blasphemies wherewith they shall blaspheme. But he that shall blaspheme against the Holy Ghost, shall never have forgiveness, but shall be guilty of an everlasting sin ». Such words are also found in Matthew and Luke.
What does the expression "blasphemy against the Holy Spirit" truly mean? The Catechism of the Catholic Church published in 1992 offers the following answer (n. 1864):
"'Therefore I say to you: Every sin and blasphemy shall be forgiven men, but the blasphemy of the Spirit shall not be forgiven'(Matthew 12: 31; cf, Mark 3: 2; Luke 12:10). There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss".
In other words, « Blasphemy against the Holy Spirit » consists in rejecting God's forgiveness until we die. It is a blasphemy against the Holy Spirit because salvation is offered to everyone by the Holy Spirit. St. John Paul II also explains the meaning of "blasphemy against the Holy Spirit" in his encyclical Dominum et vificantem (nn. 46 and 47):
"Why is blasphemy against the Holy Spirit unforgivable? How should this blasphemy be understood? St. Thomas Aquinas replies that it is a question of a sin that is unforgivable by its very nature, insofar as it excludes the elements through which the forgiveness of sin takes place. According to such an exegesis, 'blasphemy' does not properly consist in offending against the Holy Spirit in words; it consists rather in the refusal to accept the salvation which God offers to man through the Holy Spirit, working through the power of the Cross. (…)
"If Jesus says that blasphemy against the Holy Spirit cannot be forgiven either in this life or in the next, it is because this 'non-forgiveness' is linked, as to its cause, to 'non-repentance,' in other words to the radical refusal to be converted… Blasphemy against the Holy Spirit, then, is the sin committed by the person who claims to have a 'right' to persist in evil – in any sin at all – and who thus rejects Redemption. One closes oneself up in sin, thus making impossible one's conversion, and consequently the remission of sins, which one considers not essential or not important for one's life. This is a state of spiritual ruin, because blasphemy against the Holy Spirit does not allow one to escape from one's self-imposed imprisonment and open oneself to the divine sources of the purification of consciences and of the remission of sins.
"The action of the Spirit of truth, which works toward salvific 'convincing concerning sin,' encounters in a person in this condition an interior resistance, as it were an impenetrability of conscience, a state of mind which could be described as fixed by reason of a free choice. This is what Sacred Scripture usually calls 'hardness of heart.' In our own time this attitude of mind and heart is perhaps reflected in the loss of the sense of sin… Pope Pius XII had already declared that 'the sin of the century is the loss of the sense of sin,' and this loss goes hand in hand with the 'loss of the sense of God'.
"Hence the Church… constantly implores with the greatest fervor that there will be no increase in the world of the sin that the Gospel calls 'blasphemy against the Holy Spirit.' Rather, she prays that it will decrease in human souls – and consequently in the forms and structures of society itself – and that it will make room for that openness of conscience necessary for the saving action of the Holy Spirit. The Church prays that the dangerous sin against the Spirit will give way to a holy readiness to accept his mission as the Counselor, when he comes to 'convince the world concerning sin, and righteousness and judgment'."
In conclusion, let us not turn down God's forgiveness. After we die, there will be no second chance. In his goodness, God has granted us a chance to be saved. We must repent of our sins, humbly and rapidly, while we are alive, and accept the free gift of God's grace.