Marvels - Explained in the following verse. Compare 2 Samuel 7:23; Psalm 77:14.

I will do marvels - This seems to refer to what God did in putting them in possession of the land of Canaan, causing the walls of Jericho to fall down; making the sun and moon to stand still, etc. And thus God made his covenant with them; binding himself to put them in possession of the promised land, and binding them to observe the precepts laid down in the following verses, from Exodus 34:11-26 inclusive.

And he said, behold, I will make a covenant,.... Or renew the covenant before made the people had broke; which on his part was, that he would, as Moses had entreated, forgive the sin of the people, go along with them, and introduce them into the land of Canaan, and drive out the inhabitants of it before them; and, on their part, that they should avoid idolatry, and everything that led unto it, particularly making covenants, and entering into alliances with the idolatrous nations cast out:

before all thy people I will do marvels, such as have not been done in all the earth, nor in any nation; both in their passage through the wilderness, and entrance into Canaan's land, and the conquest of that; such as the earth opening its mouth and swallowing alive Korah, Dathan, and Abiram, and was a new thing God created; the smiting of the rock at Kadesh, from whence flowed waters abundantly; the healing of such as were bit by fiery serpents through looking at a serpent of brass; Balaam's ass speaking, and reproving the madness of the prophet; the division of the waters of Jordan; the fall of the walls of Jericho at the sound of rams' horns; the sun and moon standing still, until the Lord had avenged himself of his enemies:

and all the people among which thou art shall see the work of the Lord; for it should be visible, as the above things were, and plainly appear to be the Lord's doing, and not man's, being above the power of any created being to perform:

for it is a terrible thing that I will do with thee; Aben Ezra restrains this to Moses's person, and interprets this of the wonderful shining of the skin of his face, when he came down from the mount, which made the children of Israel afraid to come nigh him; and of his vigorous constitution at the time of his death, when his eye was not dim, nor his natural force abated, contrary to the nature of ancient persons: but it is better to understand it of the ministry of Moses, and of the awful things that God would do by him; or rather of the people of Israel, among whom, and for whose sake, God would do such things as should cause a panic among the nations all around them; particularly what he did for them to Og king of Bashan, and Sihon king of the Amorites, on account of which terror fell, as on the king of Moab, so on the inhabitants of Canaan; see Numbers 21:33 Joshua 2:9.

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.

34:10 Behold I make a covenant - When the covenant was broke, it was Israel that broke it; now it comes to be renewed, it is God that makes it. If there be quarrels, we must bear all the blame; if there be peace, God must have all the glory. Before all thy people I will do marvels - Such as the drying up of Jordan, the standing still of the sun. Marvels indeed, for they were without precedent, such as have not been done in all the earth; the people shall see, and own the work of the Lord; and they were the terror of their enemies: it is a terrible thing that I will do.

34:10-17 The Israelites are commanded to destroy every monument of idolatry, however curious or costly; to refuse all alliance, friendship, or marriage with idolaters, and all idolatrous feasts; and they were reminded not with idolaters, and all idolatrous feats; and they were reminded not to repeat the crime of making molten images. Jealously is called the rage of a man, Pr 6:34; but in God it is holy and just displeasure. Those cannot worship God aright, who do not worship him only.