17 That expression also of His, “He that eats My Flesh and drinks My Blood dwells in Me, and I in him,” how must we understand? Can we include in these words those even of whom the Apostle says, “that they eat and drink judgment to themselves;” when they eat this flesh and drink this blood? What! Did Judas the impious seller and betrayer of his Master (though, as Luke the Evangelist declares more plainly, he ate and drank with the rest of His disciples this first Sacrament of His body and blood, consecrated by the Lord's hands), did he “dwell in Christ and Christ in him”?
Do so many, in fine, who either in hypocrisy eat that flesh and drink that blood, or who after they have eaten and drunk become apostate, do they “dwell in Christ or Christ in them”? Yet assuredly there is a certain manner of eating that Flesh and drinking that Blood, in which whosoever eats and drinks, “he dwells in Christ and Christ in him.” As then he does not “dwell in Christ and Christ in him,” who “eats the Flesh and drinks the Blood of Christ” in any manner whatsoever, but only in some certain manner, to which He doubtless had regard when He spoke these words.
So in this expression also, “He that shall blaspheme against the Holy Ghost has never forgiveness,” he is not guilty of this unpardonable sin, who shall blaspheme in any way whatever, but in that particular way, which it is His will, who uttered this true and terrible sentence, that we should seek out and understand.
Source: Sermons on Selected Lessons of the New Testament (New Advent)