John Gill Commentary Exodus 7:5

"And the Egyptians shall know that I am Jehovah, when I stretch forth my hand upon Egypt, and bring out the children of Israel from among them." — Exodus 7:5 (ASV)

And the Egyptians shall know that I am the Lord
Jehovah, the one only true and living God; this they should know by the judgments executed upon them, and be obliged to acknowledge it:

when I stretch forth mine hand upon Egypt :
especially the last time, to destroy the firstborn:

and bring out the children of Israel from among them ;
by which it would appear that he was mightier than they, and obtained the end for which the plagues were inflicted on them.