Mentally ill isnt the right word.
If you was to say people who believe in God are naive, then yes.
Nice edit XD
Naive to believe there's something more than just what we see and our existence is a total luck of the draw? There's nothing naive in believing there's a higher power.
Now it's naive if you're indoctrinated (the actual definition) at any age into any religion and just accepting it.
It's sad that those who don't get it have to attack it when they believe it's nothing.