When I was a Christian, I was Republican. The younger me was very eager to accept pat answers to my questions. And I think that is why the Republican party appeals to some Christians. Modern Republicans have easy answers to everything and they invoke the name of god often, plus they purposely cater to fears. The same factors that attract people to fundamentalism will probably attract them to the republican party.
In some ways I could be a Libertarian. I believe that we should be able to do what we want with our own bodies, even if it is stupid. You want to do drugs, ruin your health and destroy the love of anyone who cares for you. Knock yourself out but leave me alone.
But I can't get completely behind Libertarianism because I think that the government should be responsible for some social programs.
Really, there is no party that fits all my believes so I have to look at individual candiate's platforms to decide who to vote for.
It is mainly a Christian thing due to the fact that the religious right propaganda has taken over that party. If you are a moderate conservative or libertarian who is not for a theocracy and can actually look at other sides with an open mind, it's one thing, but what the Rep party has come to represent with their current fan base is anything but moderate. There are some atheist Reps who choose to ignore a lot of the negatives, but they are relatively few.
However, I am not saying the Dems are any better. As i have said before, I have some both liberal and conservative values, even without religion, and the way many "liberal Democrats" act is just as dogmatic as the Christian conservatives in my opinion. They don't want people generalizing them, but they generalize and stereotype "conservative" anything like hell or make no real attempt at understanding their arguments.