Since the U.S. Congress is the only Federal body that can constitutionally create Federal Law in the United States of America the Supreme Court overstepped its bound by making up a Separation of Church and State rule.
The Constitution itself names the Supreme Court as the ONLY body which can interpret the meaning of the Constitution. In that specific and fully supported role, the SC interpreted the First Amendment in light of writings of the founders as to its intent and decided that a 'wall of separation' was indeed that intent. In other words, they didn't make any law - the law was already in place.