Exodus 34:10
And he said, Behold, I make a covenant: before all thy people I will do marvels, such as have not been done in all the earth, nor in any nation: and all the people among which thou art shall see the work of the LORD: for it is a terrible thing that I will do with thee.
Exodus 34:10 records God's answer renewing His covenant and favor with stiffnecked Israel after their idolatry and sin at Sinai, in response to Moses' intercession and plea for pardon and for God's presence to go with them. The covenant thus renewed is the Ten Commandments, written by the Lord Himself on the two tables of stone that Moses had prepared and carried to the summit, where Moses remained forty days and nights, miraculously sustained alone with God while no man was permitted to go up with him or approach the mount. Before all the people God promised to do marvels and great unprecedented works, such as have not been done in all the earth nor in any nation, and all the people among whom Israel dwelt would see the work of the Lord.
Those works would reveal His presence, power, majesty, and special care and love for His people. What God said He would do with Moses and Israel is called a terrible thing, meaning an awe-inspiring thing. The renewal gave Israel the pledge of God's reconciliation and restored favor, yet in conscious guilt Aaron and the people were afraid to come near the heavenly light reflected from Moses' face when he descended, so Moses put a veil upon his face when he spoke to them in the camp. This shows that there is fear in guilt and that the soul free from sin will not wish to hide from the light of heaven.
The verse shows that God is a jealous God who requires His people to keep His Ten Commandments and to keep the Sabbath He sanctified for them, the seventh day as specified in His Word. It warns against becoming careless and indifferent and thinking it does not matter what day is kept, leaving no excuse for keeping the first day of the week. The verse is given for professing Christians to study so they will act like Christians and maintain their connection with God.