Exodus 20:20

And Moses said unto the people, Fear not; for God is come to prove you, His intention was to test the disposition of their hearts, to try them out with this terrifying aspect, that His fear may be before your faces that ye sin not. By the Law is the knowledge of sin, and God wanted to manifest His glory in order to work in their hearts the proper, holy reverence which would keep them from faithlessness and disobedience.