Hebrews 12:21

"And so terrifying was the sight." It wishes to show that the New Testament also excels in relation to the Old. For indeed, even if the Old had nothing of this kind that the New has, it was to be feared in some way; however, …

While he brought out the fearsome things, he did not reveal the fruit springing from them. He did not say he "appeared," because what they saw was not the God of all in person but some impression of the divine coming.

Wonderful indeed were the things in the Temple, the Holy of Holies; and again awful were those things also that were done at Mount Sinai, "the fire, the darkness, the blackness, the tempest." For, it says, "God appeared in Sinai," and long ago were these things celebrated. The New Covenant, however, was not given with any of these things, but has been given in simple discourse by God. See then how he makes the comparison in these points also. And with good reason has he put them afterwards. For when he had persuaded them by innumerable arguments, when he had also shown the difference between each covenant, then afterwards, the one having been already condemned, he easily enters on these points also. These things, he means, are terrible; and so terrible that they could not even bear to hear them, that not even "a beast" dared to go up. For it is said, "Let not God speak, but let Moses speak unto us. And so fearful was that which was commanded, Though even a beast touch the mountain, it shall be stoned; Moses said, I exceedingly fear and quake." What wonder as respects the people? He himself who entered into "the darkness where God was," saith, "I exceedingly fear and quake." Fearful were those things, but these are far more admirable and glorious. For here there is not "darkness," nor "blackness," nor "tempest." It seems to me that by these words he hints at the obscurity of the Old Testament, and the overshadowed and veiled character of the Law. And besides the Giver of the Law appears in fire terrible, and apt to punish those who transgress. But what are "the sounds of the trumpet"? Probably it is as though some King were coming. This at all events will also be at the second coming. "At the last trump" all must be raised. But it is the trumpet of His voice which effects this. […]

He shows the superiority of the New Testament in this as well. For the Old Testament, not having anything as great as the New, was something terrifying, so that according to it not even a beast could touch the mountain (Exod. 19:12), that is, any …

And so terrible was the sight,.... Of the smoke, fire, and lightnings; or of God himself, who descended on the mount; with which agrees the Arabic version, which renders the words, "and so terrible was he who vouchsafed himself to be seen"; not in the bush burning with fire; at which time Moses was afraid to look upon God, Exo 3:6 but on Mount Sinai, when the law was given: that Moses said, I exceedingly fear and quake: perhaps he might say this, when he spake, and God answered him, Exo 19: