Luke 10:18 - Ellicott's Commentary On The Whole Bible

I beheld Satan as lightning fall from heaven. — The tense of the first Greek verb implies continuous action: I was beholding Satan as he fell... While they were working their Master had been following them in spirit, gazing, as it were, on each stage of their victorious conflict. Their triumph over the demons was the beginning and the earnest of a final conquest over Satan as “the prince of the demons.” There may, possibly, be a reference to the belief then beginning to be current among the Jews as to the fall of Satan after his creation; but the primary meaning of our Lord’s words is that he was now dethroned from his usurped dominion in the “high places” (comp. Ephesians 6:12), which symbolised the spiritual region of the soul and mind of man. The imagery reappears in a developed form in Revelation 12:9.