Luke 10:18 - Albert Barnes' Notes on the Bible

I beheld Satan ... - “Satan” here denotes evidently the prince of the devils who had been cast out by the seventy disciples, for the discourse was respecting their power over evil spirits. “Lightning” is an image of “rapidity” or “quickness.” I saw Satan fall “quickly” or rapidly - as quick as lightning. The phrase “from heaven” is to be referred to the lightning, and does not mean that he saw “Satan” fall “from heaven,” but that he fell as quick as lightning from heaven or from the clouds. The whole expression then may mean, “I saw at your command devils immediately depart, as quick as the flash of lightning. I gave you this power - I saw it put forth - and I give also now, in addition to this, the power to tread on serpents,” etc.