I was having a great discussion about this the other night in the chatroom.
Personally, I would love to live in a Christian theocracy where the laws of the land were taken right from the bible. However, I know that this is only likely to happen if the majority of people in my country vote a Christian political party into power, and even then there would likely be massive resistance to the establishment of a theocracy.
However in saying that, I don't think that Christian interests should unfairly influence the decisions of a government. A government that was truly based on Christian principles would obviously make good decisions in our eyes, such as banning abortion and gay marriage. However, if the majority of people in a nation want both legalised, I don't believe in trying to stop them through the use of the government and legal system. I know as Christians we would be more comfortable in a world that conforms with our morals, but I think if we want our morals and values to be adopted by the rest of society, the way to do it is evangelism, not lobbying the government.
If a government in a nation where Christians are not the majority is upholding laws based on Christian principles just to appease Christians, to the detriment of people who don't share those values, to me it is similar to the apartheid in South Africa, where a government representing the interests of 20% of the population was ruling the entire country.
In conclusion, I believe that religion should influence politics, but only if its influence is proportionate to its popularity.