Adam Clarke on Acts 8:10

This man is the great power of God - That is, he is invested with it, and can command and use it. They certainly did not believe him to be God; but they thought him to be endued with a great supernatural power.

There is a remarkable reading here in several MSS. which should not pass unnoticed. In ABCDE, several others, together with the Ethiopic, Armenian, later Syriac, Vulgate, Itala, Origen, and Irenaeus, the word καλουμενη is added before μεγαλη, and the passage reads thus, This person is that power of God which is Called the Great. This appears to be the true reading; but what the Samaritans meant by that power of God which they termed the Great, we know not. Simon endeavored to persuade the people that he was a very great personage, and he succeeded.