Sleepie Rosie, I just had a glance at that T-Shirt. I'm sure you wouldn't dream of wearing such a horrible thing. I know some people might think it smart or funny, but that for me is where it crosses the line. it's one thing not to want someone's religion (or indeed, beliefs generally) invading your space, but it's quite another to go out of one's way to offend their sense of decency, as a T-shirt like that does.
I quite understand people not wanting to hear about my religion and my beliefs, but at work, for example, I find myself all the time having to hear about theirs as certain people lose no opportunity to mock the religion to which I proscribe and speak almost with venom of it (i am not exaggerating here). I never bring my religion up at work unless religion and beliefs are already a topic of conversation started by someone else, and even then I rarely do. Likewise I'd expect my non-believing colleagues to at least practice their disdain behind closed doors.
Not wanting another person's religion in your face (which I accept as quite reasonable) has a corollary - not going out of your way to ridicule their beliefs. In fact, doing so only puts them in the unfair position of feeling obliged to say something about it while binding them with the restraint at the same time that if they do, they are somehow 'evangalising'
I'm sure the T-shirt was just a joke, but I hope you can see it is a tad offensive?