Then the Lord said to Moses, “Tell Aaron, ‘Stretch out your staff and strike the dust of the ground,’ and throughout the land of Egypt the dust will become gnats.” 17 They did this, and when Aaron stretched out his hand with the staff and struck the dust of the ground, gnats came on people and animals. All the dust throughout the land of Egypt became gnats. 18 But when the magicians tried to produce gnats by their secret arts, they could not.
Since the gnats were on people and animals everywhere, 19 the magicians said to Pharaoh, “This is the finger of God.” But Pharaoh’s heart was hard and he would not listen, just as the Lord had said.
In this story the plagues go up a gear, because the Egyptian magicians can’t duplicate this one.
‘…suddenly they realised that they were up against a greater power than their own. They told Pharaoh: “This is the finger of God” (verse 19).
From this point on, Pharaoh’s officials began more and more to understand the power of the God of Israel. But Pharaoh himself, in his pride, determined to oppose God.’ Tom Hale: ‘Applied Old Testament Commentary’, p.219.
Hale also includes this footnote:
‘God is so powerful that all He needs is His finger to make things happen. The expression “finger of God” (Luke 11:20) is similar to the expressions “arm of God” or “hand of God” (Exodus 9:3); it simply means “God’s action” or “God’s power.”