Thomas Aquinas devotes a question in his Summa Theologiae to “blasphemy against the Holy Spirit.” He gives more than one possible interpretation.
Three meanings have been given to the sin against the Holy Ghost. For the earlier doctors, viz. Athanasius (Super Matth. xii, 32), Hilary (Can. xii in Matth.), Ambrose (Super Luc. xii, 10), Jerome (Super Matth. xii), and Chrysostom (Hom. xli in Matth.), say that the sin against the Holy Ghost is literally to utter a blasphemy against the Holy Spirit, whether by Holy Spirit we understand the essential name applicable to the whole Trinity, each Person of which is a Spirit and is holy, or the personal name of one of the Persons of the Trinity, in which sense blasphemy against the Holy Ghost is distinct from the blasphemy against the Son of Man (Matthew 12:32), for Christ did certain things in respect of His human nature, by eating, drinking, and such like actions, while He did others in respect of His Godhead, by casting out devils, raising the dead, and the like: which things He did both by the power of His own Godhead and by the operation of the Holy Ghost, of Whom He was full, according to his human nature. Now the Jews began by speaking blasphemy against the Son of Man, when they said (Matthew 11:19) that He was “a glutton . . . a wine drinker,” and a “friend of publicans”: but afterwards they blasphemed against the Holy Ghost, when they ascribed to the prince of devils those works which Christ did by the power of His own Divine Nature and by the operation of the Holy Ghost.
Augustine, however (De Verb. Dom., Serm. lxxi), says that blasphemy or the sin against the Holy Ghost, is final impenitence when, namely, a man perseveres in mortal sin until death, and that it is not confined to utterance by word of mouth, but extends to words in thought and deed, not to one word only, but to many. Now this word, in this sense, is said to be uttered against the Holy Ghost, because it is contrary to the remission of sins, which is the work of the Holy Ghost, Who is the charity both of the Father and of the Son. Nor did Our Lord say this to the Jews, as though they had sinned against the Holy Ghost, since they were not yet guilty of final impenitence, but He warned them, lest by similar utterances they should come to sin against the Holy Ghost: and it is in this sense that we are to understand Mark 3:29-30, where after Our Lord had said: “But he that shall blaspheme against the Holy Ghost,” etc. the Evangelist adds, “because they said: He hath an unclean spirit.”
But others understand it differently, and say that the sin of blasphemy against the Holy Ghost, is a sin committed against that good which is appropriated to the Holy Ghost: because goodness is appropriated to the Holy Ghost, just a power is appropriated to the Father, and wisdom to the Son. Hence they say that when a man sins through weakness, it is a sin “against the Father”; that when he sins through ignorance, it is a sin “against the Son”; and that when he sins through certain malice, i.e. through the very choosing of evil, as explained above (I-II, 78, 1,3), it is a sin “against the Holy Ghost.”
Now this may happen in two ways. First by reason of the very inclination of a vicious habit which we call malice, and, in this way, to sin through malice is not the same as to sin against the Holy Ghost. On another way it happens that by reason of contempt, that which might have prevented the choosing of evil, is rejected or removed; thus hope is removed by despair, and fear by presumption, and so on, as we shall explain further on (Q 20,21). Now all these things which prevent the choosing of sin are effects of the Holy Ghost in us; so that, in this sense, to sin through malice is to sin against the Holy Ghost.