1 Spiritual in truth is the grace we need, in order to discourse concerning the Holy Spirit; not that we may speak what is worthy of Him, for this is impossible, but that by speaking the words of the divine Scriptures, we may run our course without danger. For a truly fearful thing is written in the Gospels, where Christ has plainly said, Whosoever shall speak a word against the Holy Ghost, it shall not be forgiven him, neither in this world, nor in that which is to come. And there is often fear, lest a man should receive this condemnation, through speaking what he ought not concerning Him, either from ignorance, or from supposed reverence. The Judge of quick and dead, Jesus Christ, declared that he has no forgiveness; if therefore any man offend, what hope has he?
Source: Catechetical Lectures (New Advent)