America was founded by a bunch of Christians, and some of the country's construction surely was influenced by that.
However America was NOT founded as a Christian state.
Where the left & right butt heads over this is usually when, for example, pro-lifers say "God clearly forbids abortion and America is a Christian state so our laws must follow the Bible", and pro-choicers say "Uh, no, this is not a Christian state, the Bible does not set our laws".
The fight has never been about the political and religious history; it's been about the meaning and implication of that history. Does the fact that Christianity played such an important historical role in European and American political history mean that it should therefore be given special legal status? For example, is it therefore okay for the government to display the 10 Commandments but refuse to display Buddhists' 5 Precepts? Where do you draw the line between historical homage and preferential treatment? Can such a line even be drawn? The anecdotes and contours of the debate have changed over the centuries but not the fundamental conflicts and tensions.
It's when you get a large number of people in public office explicitly fighting to bring their religion into the public space, to force prayer on schools, to question and be suspicious of anyone with different religions, that we get a problem.
The pushback you're seeing is the pushback against a majority trying to impose its faith on all Americans.
It's a huge stretch for you to take propaganda posters from the 50s and 60s, during the Red Scare against atheist communists, and project that upon the entire country's 200+ year history, and to assert that the POSITIVE IMPACT of that religion cannot be denied.