Calvin on Exodus 19:17-20:
We learn from these words that the prodigies were not intended to drive the people from God’s sight, and that they were not smitten with fear to exasperate and disgust them with the doctrine, but that God’s covenant was no less lovely than alarming; for they are commanded to go and “meet God,” presenting themselves with minds ready unto obedience. But this could not be unless they heard in the Law something besides precepts and threatenings. Yet in the smoke and fire, and other signs, some fear was added, in accordance with the office of the Law, because the sinner will never be capable of pardon until he learns to tremble from consciousness of his guilt, nay, until confounded with dread he lies like one dead before the tribunal of God.
Source: http://www.puritanboard.com/showthread.php/89879-No-less-lovely-than-alarming, Comment #1