Dear Fr. Fortea, can a demon cause a mental illness in a person?
Demons can tempt us, and they sometimes do so in a continuous way with great intensity. As such, a demon could provoke an obsession, phobia, depression, or another type of mental illness in a person. This 
can seriously disturb a person’s ordinary life to the point of making him unbalanced. So, yes, a demon can cause mental illness – but only if God permits it. (Of course, everything that occurs must either be willed or at least permitted by God.)
Given the internal mechanism that is used to cause temptation – that is, the infusion of a thought or image in our intellect, memory, or imagination – this modus operandi can be used in such a pernicious way that it can unbalance a person. This is well within the power of a demon. The only thing that can prevent this activity is the will of God. So does God always prevent demons from causing a mental illness in a person? Undoubtedly, no. But this is the exception, not the rule. In most cases, mental illnesses have purely natural causes.
Editor’s Note: To learn more about spiritual warfare and demonology, Catholic Spiritual Direction recommends Fr. Fortea’s excellent book Interview With An Exorcist – An Insider’s Look at the Devil, Demonic Possession, and the Path to Deliverance.
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Art for this post on whether or not demons can cause mental illness: Gargoyle 13, Gargoyles Magdalen College Oxford England, Photographed by Chris Creagh, 27 July 2009 own work, CCA-SA 3.0 Unported; detail of Padre José Antonio Fortea 2017, photographed by Elgatoconbotaselgatoconbotas, 5 May 2017 own work, CCA-SA 4.0 International, both Wikimedia Commons. Cover of “Interview with an Exorcist” used with permission, all rights reserved.
