The Unpardonable Sin (sometimes known as the Unforgivable Sin) is a concept on Christian philosophy related to sin (which is considered a type of spiritual evil, thus valid for this wiki).
It is the only sin regarded as completely unforgivable, to such an extent that committing it results in insant Damnation: though there is growing debate amongst Christians on what this implies and Universalists by default do NOT belief in this concept.
The Unpardonable Sin is said very clearly be "willful rejection of the Holy Spirit" - this usually means that in Christian views one has committed a truly unthinkable sin when one has been given a Divine Revelation (such as God appearing, performing miracles and so forth) and knowing it is the work of God, yet deliberately going against it and declaring the works of God as that of the Devil or flat-out rejecting salvation.
Although this concept is normally reserved for humans it can be argued the first being to commit the Unpardonable Sin was Lucifer the Archangel, who according to Scripture spent much of his immortal life gazing upon God yet willfully went against Him.
It is also widely believed that no "true Christain" can commit this crime as it requires someone to literally gaze upon God or His wonders and know they are real, yet deny them or try to credit God's work to Satan (knowing fine well it is not): very few people are considered capable of this level of detachment from God, the group most infamous for committing this crime were the Pharisees (mortal enemies of Jesus during his teachings).
The Unpardonable Sin is also believed by most mainstream teachers to be so grievous that anyone who has a fear of committing the sin is considered "not guilty" of the offence: to commit the sin one must be aware of God's geniune benevolence and grace, yet willfully reject or oppose it.