35 And in the narrative of the two other Evangelists, the occasion of speaking out this sentence of the blasphemy of the Spirit arose from the mention of the unclean spirit, who is divided against himself. For it had been said of the Lord, that “He cast out devils by the prince of the devils.” In that place the Lord says, that “by the Holy Spirit He casts out devils,” that so the spirit who is not divided against Himself may overcome and cast out him who is divided against himself; but that that man would abide in his perdition, who refuses through impenitence to pass over into His peace, who is not divided against Himself.
For thus runs the narrative of Mark; “Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme; but he that shall blaspheme against the Holy Ghost has never forgiveness, but shall be held guilty of an eternal offense.” When he had delivered these words of the Lord, he then subjoined his own, saying, “Because they said He has an unclean spirit;” that He might show that the cause of His saying this arose hence, because they had said that “He cast out devils by Beelzebub the prince of the devils.”
Not that this was a blasphemy which shall not be forgiven, forasmuch as even this shall be forgiven, if a right repentance follow it; but because, as I have said, there arose hence a cause for that sentence to be delivered by the Lord, since mention had been made of the unclean spirit whom the Lord shows to be divided against himself, because of the Holy Spirit who is not only not divided against Himself, but who also makes those whom He gathers together undivided, by forgiving those sins which are divided against themselves, and by inhabiting those who are cleansed, that it may be with them, as it is written in the Acts of the Apostles, “The multitude of them that believed were of one heart and of one soul.” And this gift of forgiveness none resists, but he who has the hardness of an impenitent heart.
For in another place also the Jews said of the Lord that He had a devil, yet He spoke nothing there of the blasphemy of the Holy Spirit; because they did not so bring forward the mention of the unclean spirit as that he could be shown out of their own mouths to be divided against himself, as Beelzebub, by whom they said that devils could be cast out.
Source: Sermons on Selected Lessons of the New Testament (New Advent)