And Pharaoh said, Who is the LORD, that I should obey his voice to let Israel go? I know not the LORD, neither will I let Israel go.

Wesley's Notes

5:2 Who is the Lord that I should obey his voice? - Being summoned to surrender, he thus hangs out the flag of defiance. Who is Jehovah? I neither know him nor care for him; neither value nor fear him. It is a hard name that he never heard of before, but he resolves it shall be no bugbear to him. Israel was now a despised, oppressed people, and by the character they bore he makes his estimate of their God, and concludes that he made no better figure among the gods, than his people did among the nations.

Jamieson-Fausset-Brown Bible Commentary

2. And Pharaoh said, Who is the Lord-rather "Jehovah." Lord was a common name applied to objects of worship; but Jehovah was a name he had never heard of. Pharaoh estimated the character and power of this God by the abject and miserable condition of the worshippers and concluded that He held as low a rank among the gods as His people did in the nation. To demonstrate the supremacy of the true God over all the gods of Egypt, was the design of the plagues.

I know not the Lord, neither will I let Israel go-As his honor and interest were both involved he determined to crush this attempt, and in a tone of insolence, or perhaps profanity, rejected the request for the release of the Hebrew slaves.

Matthew Henry's Concise Commentary

5:1-9 God will own his people, though poor and despised, and will find a time to plead their cause. Pharaoh treated all he had heard with contempt. He had no knowledge of Jehovah, no fear of him, no love to him, and therefore refused to obey him. Thus Pharaoh's pride, ambition, covetousness, and political knowledge, hardened him to his own destruction. What Moses and Aaron ask is very reasonable, only to go three days' journey into the desert, and that on a good errand. We will sacrifice unto the Lord our God. Pharaoh was very unreasonable, in saying that the people were idle, and therefore talked of going to sacrifice. He thus misrepresents them, that he might have a pretence to add to their burdens. To this day we find many who are more disposed to find fault with their neighbours, for spending in the service of God a few hours spared from their wordly business, than to blame others, who give twice the time to sinful pleasures. Pharaoh's command was barbarous. Moses and Aaron themselves must get to the burdens. Persecutors take pleasure in putting contempt and hardship upon ministers. The usual tale of bricks must be made, without the usual allowance of straw to mix with the clay. Thus more work was to be laid upon the men, which, if they performed, they would be broken with labour; and if not, they would be punished.