What does Hebrews 12:20 mean?

This passage refers to the time when God gave the old covenant to Israel. This happened after the exodus from Egypt, at Mount Sinai (Exodus 19:9–20). The scene at Sinai was dramatic and also terrifying. There, the people saw clouds and fire, heard trumpet blasts, and were commanded not to even touch the holy mountain. The purpose of this spectacle was to highlight the seriousness of this covenant and the holiness of the God who made it. Man's separation from God was a major teaching aspect of the law.

The point of this section of Hebrews is to continue contrasting the old covenant to the new covenant. The old covenant was inaugurated with a sense of dread, even terror, in order to convince man of his own sin and his need for God (Hebrews 9:8–12; Galatians 3:23–24). The new covenant, as future verses will explain, is something presented to man with an attitude of peace and forgiveness. This entire letter describes how the new covenant is superior to the old covenant, and that even includes the emotional impact felt as mankind is presented with each one.