What does Mark 9:17 mean?

"Mute" is from the Greek root word alalos which means unable to speak, just as we would think. This is not the same word used of the deaf man in Decapolis whose speech impediment prevented him from being understood (Mark 7:31–37). "Spirit" is from the Greek root word pneuma, which literally means wind or air movement, but is also used of non-corporeal beings, including the Holy Spirit (Mark 1:10), human spirits (Mark 14:38), and demons (Mark 1:23).

In the time of Christ, physical ailments are apparently often caused by demonic possession. Jesus clearly delineates illness caused by demons from those caused by medical issues. Today, some churches teach that demonic spirits inflict any number of specific hardships. Spirits are blamed for fatigue, sinful desires, mental illness, unemployment, and laziness. Although spiritual warfare is real (Ephesians 6:12), there is no indication in the Bible that people can be plagued by, say, a "spirit of infertility." Church leaders who convince people that they are being harassed or possessed by a demon that prevents them from finding work or making money are not teaching biblical truth. Christians cannot be possessed, and telling a young Christian he must go through an exorcism is tantamount to spiritual abuse.

In this case, and in other situations in the Bible, Jesus makes it clear that the ailment is caused by a demon. Undoubtedly, demons cause similar issues today. We need to exercise extreme caution, however, in making such a diagnosis.