24 Wherefore, neither must we imagine, as some do, that the word which is spoken against the Son of Man is forgiven, but that which is spoken against the Holy Ghost is not forgiven, because Christ became the Son of Man by reason of His assuming flesh, in which respect the Holy Ghost of course is greater, who in His Own Substance is equal to the Father and the Only-begotten Son according to His Divinity, according to which also the Only-begotten Son Himself is equal to the Father and the Holy Spirit.
For if this were the reason, surely nothing would have been said of any other kind of blasphemy, that that only might appear capable of forgiveness, which is spoken against the Son of Man, regarded only as man. But forasmuch as it is first said, “All manner of sin and blasphemy shall be forgiven unto men;” which in another Evangelist is also thus expressed, “All sins shall be forgiven unto the sons of men, and blasphemies wherewithsoever they shall blaspheme;” without doubt, that blasphemy also which is spoken against the Father is included in that general expression; and yet that alone is laid down as unpardonable, which is spoken against the Holy Ghost.
What! Did the Father also take the form of a servant, that in this respect the Holy Ghost should be greater than He? No surely: but after the universal mention of all sins and of all blasphemy, He wished to express more prominently the blasphemy which is spoken against the Son of Man for this reason, because although men should be even bound in that sin which He mentioned when He said, “If I had not come and spoken to them, they had not had sin:” which sin also in the Gospel according to John He shows to be a very grievous one, when He says of the Holy Spirit Himself, when He promised that He would send Him, “He shall reprove the world of sin, and of righteousness, and of judgment: of sin, because they believed not on Me:” yet if that hardness of the impenitent heart have not spoken a word against the Holy Ghost, even this which is spoken against the Son of Man shall be forgiven.
Source: Sermons on Selected Lessons of the New Testament (New Advent)