Christianity must be chosen, not forced, if it is to save a soul. And one can have a strong faith for oneself and believe it to be wrong to force that faith onto someone else — both because it will not save the other person’s soul, and because each individual ought to respect the choices of other individuals. We can have faith in, say, Methodism and still acknowledge it is not crazy for other people to believe differently. Per Oliver Cromwell, bear in mind we may be mistaken.
What’s more, Caesar has a different job than God does, or at least God is willing to allow experimentation in Caesar’s realm. For example, it would be odd to insist that God believes that grapes have to be planted in a certain way or that it is a sin for different people to discuss and experiment with different grape-planting techniques — or that there must be particular SEC regulations.
On the other hand, it is not wrong for Christians to push for a state that is consistent with Christian teachings, even if it does not force others to be Christians. And a Christian’s moral views can influence properly what he thinks the state’s ends (and means) must be. For example, the belief that freedom and the dignity of individuals ought to be protected by the state may have religious roots, and it is no less legitimate for that.
Indeed, a Christian can believe that X is a sin and still believe that X should not be illegal as a matter of state policy; he may even believe that a law against X is itself immoral. But it cannot be insisted that Christian leave his morals outside the voting booth.
It’s hard to look at Jesus’ life and Paul’s letters without seeing an indifference to political details, so long as there is individual freedom to act as God wants us to, toward each other and toward Him.