"Mental health problems..."
That might fit the people who put this sign up. Here, we will spell it out for you: If this atheist group had put up a sign that said "Have a happy Winter Solstice" and left it at that, there would be no uproar. But this spoiled brat group used the rest of their sign to insult all religions. This would be like some group putting up a nativity scene and then attaching a sign next to it that said "Atheism is a disgrace".
Groups should use this time to give season's greetings, not season's insults.