On Wednesday, I quoted Matthew 12:32, possibly one of the harshest and most difficult verses to understand in all of Scripture:
He who shall blaspheme against the Son of Man it shall be forgiven him, but he who shall blaspheme against the Holy Spirit shall never be forgiven, either here or hereafter.
So much so, that after using the verse in very much the same way I did (as an admonishment for anyone who would think the Holy Spirit not God), St. Ambrose feels it necessary to try to explain it:
Is an offense against the Son different from one against the Holy Spirit? For as their dignity is one, and common to both, so too is the offense. But if any one, led astray by the visible human body, should think somewhat more remissly than is fitting concerning the Body of Christ (for it ought not to appear of little worth to us, seeing it is the palace of chastity, and the fruit of the Virgin), he incurs guilt, but he is not shut out from pardon, which he may attain to by faith. But if any one should deny the dignity, majesty, and eternal power of the Holy Spirit, and should think that devils are cast out not in the Spirit of God, but in Beelzebub, there can be no attaining of pardon there where is the fulness of sacrilege; for he who has denied the Spirit has denied also the Father and the Son, since the same is the Spirit of God Who is the Spirit of Christ.
Personally, I would go farther than St. Ambrose, based on the ontology of the Holy Spirit — He is the source of our communion with God. Note how, in Psalm 50(51):11, the Prophet David begs for forgiveness after he has committed adultery with Bathsheba and murdered her husband:
Cast me not away from Your presence; and take not Your Holy Spirit from me.
The parallelism of this verse equates the very presence of God with the Holy Spirit. Therefore, if we deny the Holy Spirit, we deny our access to the very presence of God. We choose to cut ourselves off from and reject the forgiveness offered by God the Father through His Son. This is why blaspheming Christ is forgivable — we still have access to God’s forgiveness through the Spirit — but blaspheming against the Spirit is unforgivable because we have denied our access to the very presence and forgiveness of God.