He [Jehovah] said. See note on Exodus 3:7, and Compare Exodus 6:10.
Behold. Figure of speech Asterismos (App-6) for emphasis.
covenant of marvels. This covenant finds its complete fulfilment in the Apocalypse.
Related Commentaries of Exodus 34:10
Exodus 34:10
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.