“Believe in America” could be as simple as a call for the nation to buck up and find a new resolve to face its problems unafraid. However, for a culture that despairs of transcendence (its many outward appearances of religiosity often masking that despair, rather than seriously meeting it), “Believe in America” can be read as an invitation to pledge allegiance to the god of nation. This is an idolatry that a certain strand of triumphalist American Christianity– in thrall to the idea that America has God’s special favor– has aided.
The larger observation is that theological language is increasingly part of political discourse. One reason this is happening is that a large group of people see god [the small "g" is intentional] as being on their side– their party, their candidate– and so theological language seems natural and appropriate; another reason is that a different large group of people have no religious grounding, and so their deepest longings for purpose and meaning get projected into the political realm. For them, politics serves as a substitute religion.
The problem is obvious: no human being– no party of human beings, no nation of human beings– can fill the need we have for hope, for belief, for restoration. Meanwhile, parties and politicians will continue to appeal to our willingness to believe that, yes, they can deliver the new life we long for– even as we become increasingly angry when they inevitably cannot.