Right, but the problem is that you have to reconcile what your religion tells you to do (convert the world), with the historical facts on the ground. As you say, the US is a "pluralistic" society, which means it's liberal in the classical sense, so there's a separation of church of state. Thus, mainstream Christianity has to compromise with modern secular reality. Liberal Christians compromise in some ways, while conservative ones do so in others.

In neither case does Christianity add much of worth to secular humanism. The choice between them is like an aesthetic matter of taste in guiding fictions (myths) and traditions. It's largely a question of upbringing. But when you look at the different ideologies objectively or relatively neutrally, Christianity seems pretty archaic.