In describing the believer’s position, the apostle draws a contrast between where they were in the Mosaic Law. Next, he will tell them where they are now in Christianity. If these Jewish believers go back to the old system, they are returning to a place that was inaugurated by utter terror. In contrast, they leave a place of privilege and grace as exemplified by the heavenly city. God wanted the ancient Israelites to fear Him so they would not sin.
Exodus 20:20
Moses said to the people, “Do not be afraid; for God has come in order to test you, and in order that the fear of Him may remain with you, so that you may not sin”(ASV, 1901).
God came to the mountain before the people on the third day. He is always before us in every matter. God preceded Israel and waited for them on the mountain. Picture the huge volume of smoke and fire while the entire mountain was quaking. This was God’s method of drawing attention to Himself and making it clear once more that He and no other that was about to communicate His Law to His people. He was creating fear of Himself so they would not sin.