For they could not bear what was commanded: “If even an animal touches the mountain, it must be stoned.”
Here the apostle goes on to engage the professing Hebrews to perseverance in their Christian course and conflict, and not to relapse again into Judaism. This he does by showing them how much the state of the gospel church differs from that of the Jewish church, and how much it resembles the state of the church in heaven, and on both accounts demands and deserves...
Commenting on Hebrews 12:18-29
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...
Verse 20. For they could not endure that which was commanded. They could not sustain the awe produced by the fact that God uttered his commands himself. The meaning is not that the commands themselves were intolerable, but that the manner in which they were communicated inspired a terror which they could not bear. They feared that they should die.