We must not despair about anybody, as long as the patience of God leads him to repent, since God desires not the death of the wicked, but that the wicked turn from his way, and live (Ez. 33:11). He is a pagan today: how do you know whether he won’t become a Christian tomorrow? He is an infidel Jew today: what if tomorrow he believes in Christ? He is a heretic today: what if tomorrow he follows the catholic truth? He is a schismatic today: what if tomorrow he embraces the catholic peace? What if these whom you see in any sort of error and you condemn as totally hopeless, before they end this life, repent and find true life? Whoever spoke any words, in all his life in this body – whatever its length, be it with his mouth or only in thought, with an impenitent heart, against the remission of sins that is in the Church, spoke against the Holy Spirit.
Thus, if the Father, the Son and the Spirit forgive sins, why is that unforgivable impenitence said to be blasphemous to the Spirit only, as if he who is bound by the sin of this impenitence seemed to oppose the very gift of the Holy Spirit by which remission of sins is possible? Among others, these things are said to stress the inseparable operation of the Trinity. When the Father is said to do something, it must not be understood that he works without the Son and the Holy Spirit; when the Son does something, it is not without the Father and the Holy Spirit; when the Holy Spirit acts, it is not without the Father and the Son. This is well known to those who believe rightly or to those who strive to understand as they can. It was said of the Father that “he does the works”, because the origin of works is from him from whom the existence of the cooperating Persons originates: the Son is born from him, and the Holy Spirit principally proceeds from him, who begat the Son, with whom he has the same Spirit in common.
This is why the Lord Jesus cast out demons by the Spirit; not that he wouldn’t have been able to accomplish this by himself and he would have needed the Spirit to come to his aid because he was not strong enough for the deed. But the spirit divided in itself was conveniently expelled by that Spirit, whom the Father and the Son have undivided in himself in common. So also the sins, because they are not forgiven outside the Church, needed to be remitted in that Spirit in whom the Church is gathered as one.
So, if somebody regrets his sins outside the Church, but his heart is impenitent for such a great sin that is alien to the Church of God, what use is his penitence to him, if he says this one word against the Holy Spirit, through which he excludes himself from the Church, who received this gift so that the remission of sins be done in it through the Holy Spirit? Although this remission is the work of the Trinity, it is understood to belong peculiarly to the Holy Spirit.
For him who opposes this gift, so that he does not ask for penitence through it, but contradicts it impenitently, his sins become irremissible; not any sin in particular, but the despised or opposed remission of sins. Thus is the Holy Spirit spoken against, when one never comes from the dissipation to the congregation, which received the Spirit to forgive sins.
--Rabanus Maurus (d. 856), Commentary on the Wisdom of Sirach V, 1 (PL 109, col. 903-905)