Ehhh- I don't really buy that making the motto is making a law. I think strict interpretation of the law would apply if, say, the government said "you have to trust in God." But like Mike said, those words have no real bearing on what religion, if any, you as an individual practice.
My problem with your argument is that you're using the part of the Constitution that talks about laws and applying it to a policy that is not enforced by the gov't. I'll give you that it endorses monotheism, but everything else is a stretch.