Paul said it was a fearful thing to fall into the hands of the living God. Some translations use a different word than fearful, such as: dreadful; terrifying; terrible; awful. We know Paul is speaking from experience. I wonder how many have experienced this? There is no doubt in my mind that this is how those who have denied and defied God all their lives will feel when they stand before God at the end of this age to meet their fate. I wonder, though, if we can truly imagine the depths of horror these who denied God all their lives are going to experience when facing Him, knowing how wrong they were, and knowing their fate as a result of their unrelenting hard heartedness towards Him. Some say, well, it is only death; death is not punishment; death is nothing to fear; and/or they are getting what they want. I say such an attitude is untenable, since fear of death is given as the reason for man's bondage to sin, and nobody in their right mind wants to die. The other side of the coin of saying, "It is only death," has to be, "It is only life." Only life! It is only life they are giving up after all, and what a careless, cursory thing to say about the great gift God has granted us in giving us life in the first place. God sees life as so precious He gave His only begotten Son to suffer and die in our stead, that whosoever believeth in Him should not perish, but have everlasting life.
We should understand, by the word and Spirit of God, what you rightly called "the depths of horror these who denied God all their lives are going to experience when facing Him".
As Paul wrote to the Corinthians:
"For we must all appear before the judgment seat of Christ; that everyone may receive the things done in his body, according to that he has done, whether it be good or bad. KNOWING THEREFORE THE TERROR OF THE LORD, WE PERSUADE MEN; but we are made manifest unto God; and I trust also are made manifest in your consciences. " (II Corinthians 5:10-11)
in my own Christian experience, which has ranged over the course of about 33 1/2 years now, I think that I can honestly say that less than 1% of the professing Christians I've met (and I've met THOUSANDS of them) have any real fear of God themselves, and therefore they don't warn others about "the terror of the Lord" either.
Anyhow, "the terror of the Lord" is definitely a motivating factor in my own witnessing to others about Christ...and in my own witnessing about Christ TO MYSELF.
"The fear of the LORD is the beginning of knowledge: but fools despise wisdom and instruction. " (Proverbs 1:7)
"Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil. Though a sinner does evil a hundred times, and his days be prolonged, yet surely I know that it shall be well with them that fear God, which fear before him: But it shall not be well with the wicked, neither shall he prolong his days, which are as a shadow; because he fears not before God. " (Ecclesiastes 8:11-13)
"In the meantime, when there were gathered together an innumerable multitude of people, insomuch that they trod one upon another, he began to say unto his disciples first of all, Beware of the leaven of the Pharisees, which is hypocrisy. For there is nothing covered, that shall not be revealed; neither hid, that shall not be known. Therefore, whatsoever you have spoken in darkness shall be heard in the light; and that which you have spoken in the ear in closets shall be proclaimed upon the housetops. And I say unto you my friends, Be not afraid of them that kill the body, and after that have no more that they can do. But I will forewarn you whom you shall fear: Fear him, which after he has killed has power to cast into hell; yea, I say unto you, Fear him." (Luke 12:1-5)
