Grammatically, it's a title, of sorts. "Vishnu" is capitalized .. as are most others.
It's not like our deity has another moniker.
Trust me, your usage of the lower-case "g" causes me no sleepless nights. You just look like an idiot who doesn't understand rules of grammar.
Now, if you said, "Christians, why does your god do such and such?", you would get no quarrel from me. However, if you said "Christians, why does god do such and such?", then you just look like an idiot.
Not only that, but you look like a poser idiot who is trying to look hip and cool by attempting to offend a specific group of people, but, in reality, comes across as someone not all that intelligent.