Adam Clarke on Exodus 8:19

This is the finger of God - That is, The power and skill of God are here evident. Probably before this the magicians supposed Moses and Aaron to be conjurers, like themselves; but now they are convinced that no man could do these miracles which these holy men did, unless God were with him. God permits evil spirits to manifest themselves in a certain way, that men may see that there is a spiritual world, and be on their guard against seduction. He at the same time shows that all these agents are under his control, that men may have confidence in his goodness and power.