Chuck Colson has an interesting article on the culture in America, in both its changes and the Christian interaction with it (a topic he has covered before).  He notes the statement by then-Supreme Court Justice William Douglas:

We are a religious people whose institutions presuppose a Supreme Being…When the state encourages religious instruction or cooperates with religious authorities…it follows the best of our traditions. For it then respects the religious nature of our people and accommodates the public service to their spiritual needs.

This case was followed a few years later by court rulings banning public prayer and Bible reading in schools.

How can — or should — Christians engage the culture in America?

Is “faithful presence” enough for Christians even as the forces of secularization grow stronger and more militant? Do we say nothing when, for perhaps the first time in our nation’s history, the administration seeks, as a matter of policy, to redefine “freedom of religion”?…

And we are to disengage from politics?

Absolutely not. We cannot do so and remain faithful to the command to love God with our whole being and our neighbors as ourselves.