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.
Indulgence
"When a person sins, he acquires certain liabilities: the liability of guilt and the liability of punishment." 
A mortal sin (one that is grave, or serious, in nature and is committed knowingly and freely) is equivalent to refusing friendship with God and communion with the only source of eternal life. The loss of eternal life with God, and the eternal death of hell that is the effect of this rejection, is called the "eternal punishment" of sin. The Sacrament of Penance removes the guilt and the liability of eternal punishment related to mortal sin. "Though your sins are like scarlet, they shall be white as snow; though they are red like crimson, they shall become like wool."