Heb 12:21

“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 of those animals that the people had. But the New Testament, possessing all perfection, contains nothing fearful; on the contrary, everything in it is merciful. "That Moses also said: 'I am in fear and trembling.'" If the great Moses, who entered the cloud, felt this way, then how much more so the people?”

“703. – From all this he draws the conclusion, namely, that they were terrifying things, because not even the beasts were spared; hence he says, and so terrible was the sight. This indicates the difference between the New and Old Testaments, because the Old Testament was given in terror to terrify the hearts of Jews, who were prone to idolatry; but the New was given in love: 'You have not received the spirit of slavery again in fear, but you have received the spirit of the adoption of sons, whereby we cry Abba: Father' (Rom. 8:15). Hence Christ did not begin His preaching with fearful things, but promised the kingdom of heaven: 'Do penance, for the kingdom of heaven is at hand' (Mt. 3:2); 'The law of clemency is on his tongue' (Pr. 31:26). 704. – Then when he says, Moses said: I am frightened, and tremble, he deals with the lawgiver's fear, namely Moses: 'For the law was given by Moses' (Jn. 1:17). Now if Moses himself in giving the Law was so frightened as to say, I am frightened inwardly and I tremble outwardly, and he was more perfect than the rest, this was a sign that the Law was terrifying even to the perfect: because it did not give grace but merely disclosed guilt. Hence, it was a heavy yoke of which Peter (Ac. 15:10) says: 'which neither we nor our fathers were able to bear.' But Christ's law is a sweet yoke, because 'the charity of God has been poured forth into our hearts by the Holy Spirit who has been given to us' (Rom. 5:5). Hence, it should be noted that the words the Apostle uses here are not found in Exodus (20). Perhaps he took them from Exodus (4:10), where Moses was frightened at the sight of the burning bush and said: 'I am not eloquent from yesterday or the day before'; after which he said, in deed at least, if not in word: I am frightened and tremble.' Or perhaps the Apostle was using another version which we do not have. But it all shows that the Old Law was a law of fear.”

“the sight--the vision of God's majesty. quake--Greek, "I am in trembling"; "fear" affected his mind: "trembling," his body. Moses is not recorded in Exodus to have used these words. But Paul, by inspiration, supplies (compare Act 20:35; Ti2 3:8) this detail. We read in Deu 9:19, Septuagint, of similar words used by Moses after breaking the two tables, through fear of God's anger at the people's sin in making the golden calves. He doubtless similarly "feared" in hearing the ten commandments spoken by the voice of Jehovah.”

“"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, the New, having perfection, is gentle and without terror. "that Moses said, "I am exceedingly afraid and trembling."" This was also said by the one who was about to enter into darkness.”