Kitto's Popular Cyclopedia of Biblial Literature
Blasphemy signifies a false, irreverent, injurious use of God's names, attributes, words, and works. Whenever men intentionally and directly attack the perfections of Jehovah, and thus lessen the reverence which others entertain for him, they are blasphemers.
By the Mosaic law blasphemy was punished with death (Leviticus 24:10-16); and the laws of some countries still visit it with the same punishment. Fines, imprisonment, and various corporal inflictions are annexed to the crime by the laws of Great Britain. It is matter, however, of sincere satisfaction, that there are very few instances in which these enactments require to be enforced.
Much has been said and written respecting the blasphemy against the Holy Ghost, usually but improperly denominated the unpardonable sin against the Holy Ghost. Some refer it to continued opposition to the Gospel, i.e. obstinate impenitence or final unbelief.
But we object to this opinion, because it generalizes the nature of the sin in question. On the contrary, the Scripture account narrows it to a particular sin of a special kind, discountenancing the idea that it is of frequent occurrence, and marked by no circumstances of unwonted aggravation. Besides, all the notices which we have refer it not so much to a state of mind as to the outward manifestation of a singularly malignant disposition by the utterance of the lips.
The occasion on which Christ introduced his mention of it (Matthew 12:31, etc.; Mark 3:28, etc.), the subsequent context, and, above all, the words of Mark 3:30 ('because they said, He hath an unclean spirit') indicate, with tolerable plainness, that the sin in question consisted in attributing the miracles wrought by Christ, or his Apostles in His name, to the agency of Satan. It was by the power of the Holy Ghost, given to the Redeemer without measure, that he cast out devils: and whoever maligned the Savior by affirming that an unclean spirit actuated and enabled him to expel other spirits, maligned the Holy Ghost.
It is difficult to discover the 'sin unto death,' noticed by the Apostle John (1 John 5:16), although it has been generally thought to coincide with the blasphemy against the Holy Spirit; but the language of John does not afford data for pronouncing them one and the same. The first three Gospels alone describe the blasphemy which shall not be forgiven: from it the 'sin unto death' stands apart.
Kitto, John, ed. Entry for 'Blasphemy'. "Kitto's Popular Cyclopedia of Biblial Literature". https://www.studylight.org/encyclopedias/eng/kbe/b/blasphemy.html.