Eternal sin
However, the Church further believes there is no offence, however serious, that cannot be taken away by Baptism, or absolved from in the Confessional—that no one, however wicked and guilty, may not confidently hope for forgiveness. The Catechism says that Christ desires "the gates of forgiveness should always be open to anyone who turns away from sin." As did St Augustine, the Catholic Church today teaches that only dying unrepentant for one's sins is the only unforgivable sin. Indeed, in "Dominum et vivificantem" Pope John Paul II writes "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", and "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. This means the refusal to come to the sources of Redemption, which nevertheless remain "always" open in the economy of salvation in which the mission of the Holy Spirit is accomplished."
Thou shalt not bear false witness against thy neighbour
To keep “a clear conscience toward God and toward men”(Acts 24:16), Christians must follow Christ's example “to bear witness to the truth.”(John 18:37) The Christian is not to “be ashamed then of testifying to our Lord.”(2 Timothy 1:8) In situations that require witness to the faith, the Christian must profess it without equivocation. Christian witness to the Gospel and the obligations that flow from it are an act of justice that establishes the truth or makes it known. Catholic teaching regards martyrdom as the supreme witness given to the truth of the faith: it means bearing witness even unto death.
Christ's disciples have "put on the new man, created after the likeness of God in true righteousness and holiness."(Ephesians 4:24) By "putting away falsehood," they are to "put away all malice and all guile and insincerity and envy and all slander." (Ephesians 4:25, 1 Peter 2:1)
Public statements contrary to the truth take on a particular gravity. In court it becomes false witness. (Proverbs 19:9) False statements under oath are perjury. Acts such as these contribute to condemnation of the innocent, exoneration of the guilty, or the increased punishment of the accused. (Proverbs 18:5) These are great sins, because they gravely compromise the exercise of justice and the fairness of judicial decisions.