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 hath no forgiveness; if therefore any man offend, what hope has he?
Cyril of Jerusalem, Cyril of Jerusalem, The Catechetical Lectures of S. Cyril 20 (Nicene and Post-Nicene Fathers, Series 2, Vol. 7)