Many would disagree, and my guess would be that as many, if not more, would enthusiastically agree the statement above.
Mental illness doesn't allow you to think intelligently or rationally. Actions following out of an unstable thought process are truly playing out on the flip-side of sane behavior, too. So another reason to support the notion that those who would believe in a god have a mental illness. I would tend to think (of course) that the overwhelming majority of believers do not pose a threat to us; rather are just annoying as hell. But should believers really be exempt from being considered mentally ill?
Our current understanding of how the brain functions has shown us that mental illness and the delusions associated with some mental illnesses can be potentially harmful to those suffering from them and to others who are in close proximity to experience that disassociation with them. Our society recognizes that we must take care (although frequently, not) of those vulnerable due to their inability to be rational when the delusions take over where reality should exist.
But for the sake of this argument now, I would say the overwhelming majority are just suffering from a conscious and intentional decision to hold that their belief is just a strong conviction despite strong evidence to the contrary. Exactly what the definition of delusion is, but in their case the delusion is one they welcome or one that has become comfortable for them because of childhood indoctrination; too comfortable or safe to let go of or challenge.
Challenging that strongly held conviction (which we as skeptics or atheists frequently do) that their delusion is real gives them the opportunity to double-down on that delusion. The stronger their reaction, the stronger hold that delusion has on them. And they will hold and fight that their position remains the correct one. As do we, I might add. But our position is always the one that will adapt with conclusive evidence. Something believers never do, even when presented with things called conclusive evidence or at something, at the very least, that should make them want to investigate further. Putting their beliefs on hold for just a moment to investigate the validity of something... unheard of; dare I say something that never happens? I've never experienced it. Please do tell me when it does.
Based on the definition of what delusions are, I stand by my interpretation when I call out all those who would say they believe in a god. As much as I love or like you as a person, or care about you as a human being, if you believe there is a god based on what we know so far about jesus christ or god... I think you are deluded. And I know there are close friends who would be upset (actually have been upset with me because I called them deluded) when they read this piece, but I would ask them to read their dictionaries and tell me I'm incorrect in my thought process. Again, this is not an assault on their character or our friendship rather on their thought process about an idea about a god. So cut me some slack... would ya? Your position on this subject is the only reason I even question it.