What does Acts 8:10 mean?

Simon is a miracle-worker in the city of Samaria. The Samaritan people haven't had opportunity to truly worship God since the time of Solomon's son (1 Kings 12), so it might be understandable they don't recognize Simon has received his power from demons.

Despite what naturalists may want us to believe, supernatural powers do exist. Such power can only come from a supernatural source: God or demons. Jesus warned that as the end times approach, "false christs and false prophets will arise and perform great signs and wonders" (Matthew 24:24). Paul will warn the Thessalonians that "the coming of the lawless one is by the activity of Satan with all power and false signs and wonders" (2 Thessalonians 2:9). Moses dealt with demon-powered magicians in Egypt (Exodus 7:22). The witch at En-dor, despite being surprised that Samuel actually showed up, must have had a reputation for divination if King Saul's servants knew of her (1 Samuel 28).

The Bible gives us ideas as to how to tell if someone is performing a miracle through demons or the Holy Spirit. The first is, which kingdom does the miracle serve? Demons will not expel demons, and Jesus-followers filled with the Holy Spirit will not serve Satan (Mark 3:23–27). Another way is by what the person says. Paul will write the Corinthians, "Therefore I want you to understand that no one speaking in the Spirit of God ever says 'Jesus is accursed!' and no one can say 'Jesus is Lord' except in the Holy Spirit" (1 Corinthians 12:3). This means more than just the words, however. The sons of Sceva try to expel a demon "by the Jesus whom Paul proclaims" (Acts 19:13). The demon responds, "Jesus I know, and Paul I recognize, but who are you?" (Acts 19:15). The possessed man then beats up the so-called exorcists and throws them out of the house (Acts 19:13–16). The Holy Spirit goes to whom He will, not those who just want to use Him.