But the LORD hardened Pharaoh's heart, so that he would not let the children of Israel go.

Matthew Henry's Concise Commentary

10:12-20 God bids Moses stretch out his hand; locusts came at the call. An army might more easily have been resisted than this host of insects. Who then is able to stand before the great God? They covered the face of the earth, and ate up the fruit of it. Herbs grow for the service of man; yet when God pleases, insects shall plunder him, and eat the bread out of his mouth. Let our labour be, not for the habitation and meat thus exposed, but for those which endure to eternal life. Pharaoh employs Moses and Aaron to pray for him. There are those, who, in distress, seek the help of other people's prayers, but have no mind to pray for themselves. They show thereby that they have no true love to God, nor any delight in communion with him. Pharaoh desires only that this death might be taken away, not this sin. He wishes to get rid of the plague of locusts, not the plague of a hard heart, which was more dangerous. An east wind brought the locusts, a west wind carries them off. Whatever point the wind is in, it is fulfilling God's word, and turns by his counsel. The wind bloweth where it listeth, as to us; but not so as it respects God. It was also an argument for their repentance; for by this it appeared that God is ready to forgive, and swift to show mercy. If he does this upon the outward tokens of humiliation, what will he do if we are sincere! Oh that this goodness of God might lead us to repentance! Pharaoh returned to his resolution again, not to let the people go. Those who have often baffled their convictions, are justly given up to the lusts of their hearts.