It is said in Parshas Va'era (Exodus 7:5): "And Egypt shall know that I am the Lord when I stretch out My hand over Egypt and bring the Israelites out from among them." It is explained that the purpose of the Lord stretching out His hand over Egypt during the ten plagues and bringing the Israelites out from among them was to bring Egypt to the realization that 'I am the Lord'—to faith and recognition in the Lord and His divinity.

We must be precise with this verse, as it was not sufficient for it to say 'And Egypt shall know that I am the Lord when I stretch out My hand over Egypt,' until it added and emphasized, 'and bring the Israelites out from among them.' This implies that this goal of Egypt's recognition of the Lord would not be achieved until the completion of the stretching out of the hand over them, with the full count of the ten plagues, at which point the Israelites would leave from among them. But before that, in the midst of the stretching out of the hand, they would not yet recognize the Lord.

However, this requires examination, as from the language of several of the plagues, it seems otherwise:

For it is said before the coming of the plague of blood (Exodus 7:17): "Thus says the Lord, 'By this you shall know that I am the Lord: behold, I will strike with the rod that is in my hand upon the waters which are in the river, and they shall be turned to blood.'" And before the removal of the plague of frogs: "And Moses said to Pharaoh, 'Glorify yourself over me: when shall I pray for you, and for your servants, and for your people, to destroy the frogs from you and your houses, so that they remain only in the river?' And he said, 'Tomorrow.' And he said, 'According to your word, that you may know that there is none like the Lord our God.'" (Exodus 8:5-6). In the plague of lice, we see how this understanding already penetrated even among the magicians, upon whose sorcery Pharaoh relied when he said to Moses and Aaron, 'You are doing this by sorcery' (Rashi, Exodus 7:22), as it is said in the plague of lice: "Then the magicians said to Pharaoh, 'This is the finger of God.'" (Exodus 8:15). Again, in the warning of the plague of wild beasts: "And I will set apart on that day the land of Goshen, in which My people dwell, that no swarms of wild beasts shall be there, that you may know that I am the Lord in the midst of the earth." (Exodus 8:18). And similarly in the plague of hail: "For this time I will send all My plagues upon your heart, and upon your servants, and upon your people, that you may know that there is none like Me in all the earth." (Exodus 9:14).

Thus, it requires explanation, as since already with the plague of blood the Egyptians reached this realization, what was added again with the plague of frogs? And since they reached it with the plague of frogs, what was added with the plague of lice, and so on. Indeed, perhaps it must be said that there are different levels and stages to this recognition, and with each plague, the Egyptians' knowledge reached a higher level.