Imagine the uproar if he'd said Britain is a white country, or a heterosexual country. That's the point. It doesn't matter what his personal beliefs are, but if he says "we're a Christian country" he's basically saying that if you're not a Christian then you don't really belong here. You only have to look at this to see that, although Christianity is in the majority at about 60%, that still leaves 40% of people that aren't Christians, most of which are either not religious or didn't state their religion.

Of course, you could argue that it's no different to if he'd stated that Britain is an English speaking country. I can see that logic. It is a fair point. But language seems different somehow. It's more basic, more fundamental. The point is, it's much better if a country can all understand each other by being able to speak the same language. But we don't all need to believe in the same invisible wizard in the sky.