What was a sin against the Holy Ghost and why was it unpardonable?
It was refusing to allow the Holy Ghost to effect the second birth. Since
baptism by whatever means into the
Holy Ghost was the only means of redeeming
sins against the
Father or the Son, the refusal or prevention of baptism meant there could be no forgiveness. It was the only route so could not be avoided if sin was to be pardoned. An offence' committed against the third limb of the Godhead barred the door to forgiveness, in this life or that to come.
To sin against the Holy Ghost was to block the path by which the door of
heaven was to be reached.