Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men. Matthew 12:31. AG 215.1
Whatever the sin, if the soul repents and believes, the guilt is washed away in the blood of Christ; but he who rejects the work of the Holy Spirit is placing himself where repentance and faith cannot come to him. It is by the Spirit that God works upon the heart; when men willfully reject the Spirit, and declare it to be from Satan, they cut off the channel by which God can communicate with them. When the Spirit is finally rejected, there is no more that God can do for the soul.... AG 215.2
It is not God that blinds the eyes of men or hardens their hearts. He sends them light to correct their errors, and to lead them in safe paths; it is by the rejection of this light that the eyes are blinded and the heart hardened. Often the process is gradual, and almost imperceptible. Light comes to the soul through God's word, through His servants, or by the direct agency of His Spirit; but when one ray of light is disregarded, there is a partial benumbing of the spiritual perceptions, and the second revealing of light is less clearly discerned. So the darkness increases, until it is night in the soul.... AG 215.3
It is not necessary for us deliberately to choose the service of the kingdom of darkness in order to come under its dominion. We have only to neglect to ally ourselves with the kingdom of light.... The most common manifestation of the sin against the Holy Spirit is in persistently slighting Heaven's invitation to repent. Every step in the rejection of Christ is a step toward the rejection of salvation, and toward the sin against the Holy Spirit.71The Desire of Ages, 322-324. AG 215.4
When the soul surrenders itself to Christ, a new power takes possession of the new heart. A change is wrought which man can never accomplish for himself. It is a supernatural work, bringing a supernatural element into human nature. The soul that is yielded to Christ becomes His own fortress, which He holds in a revolted world, and He intends that no authority shall be known in it but His own. A soul thus kept in possession by the heavenly agencies is impregnable to the assaults of Satan.72The Desire of Ages, 324. AG 215.5