A common defense of theism and religion in general is that, while irrational faith-based belief has no grounding in reality, it’s fine because it helps people cope with real life.
The most obvious response is to point out that religion hurts people besides the believer:
On May 21, 2005, LaRose haunted by what appears to be a sad and hard life began drinking heavily. Depressed over her father’s recent death, LaRose in an attempt to take her life, swallowed eight to 10 prescription muscle relaxers. Failing to kill herself, Colleen LaRose was now at a crossroads in her life. After months of receiving counseling for depression and alcoholism, LaRose apparently found spiritual rebirth in the form of Islam. Unfortunately, the brand of Islam that gave her purpose to live was a brand that advocated death to America, the West and Israel.
More subtly, but just as valid, is the observation that religion doesn’t even necessarily make the believer happier.
While the most common “benefit” of belief is “coping with death,” studies have shown religious types actually have the hardest time coping with their own death.
But if that didn’t convince you, read on.
People I know in real life have been very much hurt by their religion. A family member, for example, is incapable with dealing with human sexuality in any but the most conservative context, to the detriment of this person’s personal life.
A number of friends of mine have broken up with significant others because they found they loved them “more than they loved Jesus.” (This kind of blows my mind, since that means their closest relationship is with an imaginary, Aramaic-speaking friend who wants to send most people to hell.)
As recounted in The God Virus, religion can even make priests miserable. The author of the God Virus tells of a priest so overcome with Catholic-inspired guilt over his occasional masturbation — that he ends up hating himself for it, unable to think of much else.