Some ideas touch on just about every issue.  The existence of God and how we respond to that existence has enormous significance.  Some believe that God is just a human construction, an “opiate of the people”, as Karl Marx suggested.  Many others not only believe that God exists, but that it is our calling to serve Him.

We all order our lives around how we answer this question.

Our founders were familiar with state religion.  Europe in the 18th century was enmeshed in various wars and intrigues caused by power struggles between monarchs, popes, ethnic and religious groups.  They saw clearly the dangers in having the state impose religion, and also in having religion take power.  Their ideal was a minimal, secular government that did not have the power to either impose religion, nor interfere with the faith of citizens. They forbade the state from adopting any particular religion in article VI of the constitution, and emphasized that position in the fist amendment, which starts: “Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;…”

Unfortunately, we have strayed from that ideal, and largely because the state is so involved in so many aspects of our lives.  For instance, I doubt that the founders would have been comfortable with the idea of prayers in state-run public schools, but I am also fairly sure that they would have opposed the prohibition of religious expression in those same schools, as outlined in this article:
http://politicaloutcast.com/2013/05/liberal-myth-of-separating-god-from-the-state

The most frequent justification for interfering with religious expression is that someone, somewhere will be offended, but we do not have a right to be un-offended.  It is precisely the offensive speech that is protected.  It is the offensive expression that needs protection.  The SCOTUS got it right with the Nazis in Skokie.

The words “wall of separation” does not appear in our constitution.  The key phrase is “Congress shall make no law”.  Congress is powerless to interfere with religious expression, and with free speech.