Eternal sin
Thomas Aquinas lists, or has responded to, six sins that supposedly go against the Holy Spirit:
Thomas Aquinas explains that the unforgivability of blasphemy against the Holy Spirit means that it removes the entrance to these means of salvation; however, it cannot hinder God in taking away this obstacle by way of a miracle.
Christianity and Judaism
In Christianity, forgiveness by God is promised to the repentant even though the wronged party has not forgiven the offender: "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness." (1 John 1:9 ) Jesus, however, requires his disciples to forgive others if they want to be forgiven themselves. Matthew 6:14,15, which follows the Lord's Prayer, says "For if you forgive men when they sin against you, your heavenly Father will also forgive you. But if you do not forgive men their sins, your Father will not forgive your sins." Forgiveness is not an option to a Christian, rather one must forgive to be a Christian.