Slippery slope? Oh, I've seen this somewhere...now where was it...?
"If we let fags marry, folks'll wanna marry thurr dawgs! An' li'l kids, an' inanimate objects! Izza slip'ry slope! Hur hur hur!"
Such a shiny mirror, isn't it?
Anyways, back on topic. Just how do you plan on making sure everyone's Christian? And your particular brand of Christian, no doubt? Random piety checks? Full soul cavity searches? Maybe sin-sniffing dogs? The truth is a theocracy never works when it comes to stamping out all other religions. Sure, it does a bang up job oppressing other religions, but there are still Christians in Iran.
When Christianity becomes actively enforced, what will you do to people of other faiths? You fail to see that your religion is yours because it works for you. It offers you the answers you need. But it is the height of arrogance to assume that it holds the answers to everyone else's life. Christianity calls to you, so good for you. But what of those for whom Hinduism, or Islam, or Jainism is the answer? Oh, they go to hell, do they? So what do you do? Have full on conversions at gunpoint? Trust me, people would rather commit suicide then convert. I know this because it has happened. A lot. And the religion they were being forced into was--wait for it!--Christianity. Think about that. They were willing to damn their souls to whatever perdition they believed in, throw the gift of life back in the face of their patron creator, then turn their back on them and disgrace everything their ancestors stood for. Can you say the same? Somehow, I doubt your resolve would hold.
And why does it have to be a Christian theocracy? Jews don't rate, I suppose? There's no way you'd let Muslims take over, or Hindus. Buddhists wouldn't want the world. But still, why Christians. Why not Odinists or Shintoists?
Why do you get the brass ring? Morality? World domination, by it's very definition, is an immoral act. So again, why you?