The sons of Sceva (Acts 19) went around regularly driving out evil spirits. They were apparently known for practicing spiritual power apart from knowledge of Jesus and the true God. Did other people have real spiritual power in the ancient world apart from God? If so, how is that possible, and does this still happen today?

If you’re reading this blog, chances are you live somewhere with good access to technology. It’s easy to become so impressed by–or dependent upon–human technological achievements that we forget that the true powers at the heart of existence are spiritual powers.
This is what I mean. Christians believe that the Bible is an authoritative and true account of who God is, what he is like, and what he has taught. The various writers of the books of the Bible assume that God is creator of everything; in the New Testament we learn that Jesus, who is both fully God and fully a human being, upholds all of existence (Colossians 1:15-17). This means that it is God’s spiritual power that is ultimately responsible for everything that exists.
The biblical writers assume some other important things related to this question. They assume that we live in a world that is “fallen,” that is, a world in which things are not as they ought to be. In such a world, powerful and good things can be twisted toward evil. Spiritual power is one of those things. The biblical writers assume that this happens and that ultimately there is a host of spiritual forces, represented and led by Satan (often called the Deceiver or the Adversary), who are able to work in, through, and against people. It’s important to note here that this isn’t a sort of yin-yang opposition, where evil is thought to be the necessary and “equal” opposite of good; rather it’s an assumption that spiritual power is a real thing, a real way of “being alive,” and some evil beings (and people influenced by those beings one way or another) can access and use it.
Ultimately, the biblical writers also believe that God is always in control, and none of this catches him off guard. But that’s a deep topic for a different post.
Back to this question: for the biblical authors, then, it didn’t strike them as strange that some people used spiritual power apart from God’s direction or guidance. I’ll just give one biblical example of this. In the Old Testament historical book of 1 Samuel, chapter 28, there is a king named Saul who visits someone called “the witch of Endor.” She is better described as a “medium,” someone who, in the ancient world, was able to come into contact with the spiritual realms (apart from the true God). According to the story, this medium was able to call back the spirit of Samuel, a great prophet, from the dead, and Saul spoke with him. (Samuel reminded Saul of God’s judgment on his unfaithfulness!)
Finally, does this still happen today? I am convinced that it does. Talk to enough people about this and you’re sure to meet people who will describe having experiences with spiritual forces that they can’t explain. Christians believe that the spiritual forces of evil are alive and well and active in the world, and I believe that some people end up being influenced by–or controlled by–these forces.
However, there are three important things I should note here. First, Christians also believe that Jesus is king and has conquered sin, death, and the forces of evil. So we live in a world in which God already reigns but has yet to fully “consummate” his kingdom, or bring it to completion. We live in an in-between time of tension, but we should not fear evil forces as though they could ever actually “win”–they have already lost. Second, if you feel very skeptical about all of this, I recommend Craig Keener’s book Miracles. It is a philosophical and evidential look at the reality of miracles that should help you gain perspective on the present-day reality of spiritual power.
Finally, I want to end by encouraging you to become more and more fascinated by, and focused on, Jesus, rather than questions about spiritual powers. Jesus is the source of true life. He is the truest human who has ever lived and it is only by knowing, loving, being known by, and following him that we can discover what it means to really be alive. Knowing Jesus is the beginning of wisdom, and it is only from him–by means of the Holy Spirit–that we can understand and live by spiritual power.