In Deuteronomy 5:4, Moses reminded Israel that when the Lord first spoke The Ten Commandments, He spoke them to the whole nation, and they heard His voice from the mountain.
Then in Deuteronomy 5:6-21 Moses reiterated those awesome commandments. As you read the text, think of God declaring these things from the mountain and Israel hearing a loud voice (v. 22), emanating from the mountain, and seeing darkness and fire.
With such a display, the Lord God wanted them to know how serious these commandments were, and that they had to keep them.
Truly, anything God says, we must take seriously or suffer the consequences.